Patrick Oladimeji
email: p.oladimeji@swansea.ac.uk

Harold Thimbleby
email: h.thimbleby@swansea.ac.uk

Anna Cox
email: anna.cox@ucl.ac.uk

A performance review of number entry interfaces

Keywords: Number entry interfaces, number entry error, user interface performance, safety critical devices

Number entry is ubiquitous and there are several ways a number entry interface can be designed. Until recently, research has been focused mainly on one type of number entry interface: the numeric keypad. Various factors such as the range of values, and the space available for the design allows for several alternative interfaces to be used for number entry. In the design of medical devices such as those used for controlled drug delivery, accurate and timely entry of numbers are required in order to reduce any risk of harm to patients. This paper reviews five number entry interface styles and reports the result of an experiment conducted to evaluate the performance differences of the interfaces based on numbers used in infusion therapy in a hospital. The result shows a significant effect of interface style on speed and accuracy

Introduction

Number entry has been a part of human culture since humans learned to count and the use of numbers is evident in spoken languages around the world. Number entry interface design dates back as far as 2nd century B.C. as seen in the Salamis Tablet, an ancient Greek counting board [12, p.300]. Much later, starting in the mid 17th century, the advent of a series of mechanical calculators such as Pascal's Calculator and later, the Arithmometer, brought about a series of different design options for interacting with numbers and designing modern number entry user interfaces. Most of these interfaces are still in use today in interactive devices-although implemented in a variety of ways that account for technological advances both in software and hardware. Tasks involving entering numbers are extremely common. For instance we enter or select numbers at the cash machine, we enter, select or modify numeric values on our microwave ovens to specify time, and we often change the volume on our music player. While performing any of these tasks, the user might be oblivious to the number entry aspect of the task, after all, you only wish to withdraw some money from the bank, warm up your food or increase the volume of music. Number entry is usually such a subtask in achieving a more primary goal that it is hardly noticed and very often perceived as trivial.

The reader could probably think of more than one type of interface for performing these tasks. For instance, a cash machine might use a 12-key numeric keypad, a microwave might use a dial and the music player might use a slider. There are several ways a number entry interface might be designed. Despite dating back many years, until recently, research on number entry has failed to identify a classification of number entry interfaces, and a review of the performance of the different styles of interfaces that might be beneficial to designers of interactive systems.

The most common number entry interface is the numeric keypad. It is found on telephones, calculators and keyboards. Its popularity is not surprising since it provides a direct mapping between interaction input and output and allows sequential entry of the digits that constitute the intended number much like spoken western language. Several constraints, factors and requirements in certain contexts may however restrict the adoption of a type of number entry interface. Examples of such constraints might be space requirements or footprint of the device in question or the range and precision of values intended to be addressed in the host application.

In the design of medical devices, these constraints become more compelling and designers ought to be able to make number entry interface design decisions with a clear understanding of the strengths and weaknesses of an interface. The use of many medical devices involves entry of numeric data that represent drug doses, duration of therapy or frequency of therapy. According to a report in 2007, about 7000 medicine doses are administered each day in each hospital in England and Wales [START_REF]Safety in doses: medication safety incidents in the NHS[END_REF]. Some drugs have to be administered intravenously due to the treatment requirements of patients. This sometimes involves multiple intravenous drugs to be administered simultaneously [START_REF] Keohane | Intravenous medication safety and smart infusion systems: lessons learned and future opportunities[END_REF]. Devices such as infusion pumps, used for controlled delivery of drugs in hospitals, require timely and accurate programming in order to avoid patient harm [START_REF]NPSA:Reducing harm from omitted and delayed medicines in hospital[END_REF]. Setting up an infusion pump requires entering numbers that correspond to the rate of infusion, the volume to infuse and duration of the infusion. Many adverse incidents in hospitals have been as a result of number entry errors in programming infusion pumps [START_REF]ISMP: Double key bounce and double keying errors[END_REF][START_REF] Vicente | Programming errors contribute to death from patient-controlled analgesia: case report and estimate of probability[END_REF][START_REF] Westbrook | Errors in the administration of intravenous medications in hospital and the role of correct procedures and nurse experience[END_REF].

This paper presents the results of an empirical evaluation of five different number entry user interfaces using a customisable high-fidelity prototype. Our aim was to explore the performance difference across these interfaces with the intent of providing a summary of tradeoffs involved in choosing to implement one of the styles of interface over another.

2

Related work

The majority of research in number entry interface performance has mainly obtained performance metric on a variety of configurations of the numeric keypad. Early research by Deininger [START_REF] Deininger | Human factors engineering studies of the design and use of push button telephone sets[END_REF] in the design of telephone keypads explored the performance differences of 16 layouts and the effects of keying behaviour of users on the keying entry speed. Deininger's experiment found that the entry speed was dependent on the participant's strategy for reading the numbers. Participants who memorized the numbers before starting the keying sequence, performed significantly better than those who referred back to the number during entry. Further experiments on the effect of keypad layout by Conrad and Hull [START_REF] Conrad | The preferred layout for numeral data-entry keysets[END_REF] initially suggested that the 3 x 3 grid of telephone keypad layout with 1, 2, 3 at the top was more accurate than the calculator layout with 7, 8, 9 at the top. Marteniuk et al. [START_REF] Marteniuk | Are there task specific performance effects for differently configured numeric keypads[END_REF] later found that performance differences between different keypad layouts based on the two popular telephone and calculator layouts were as a result of the placement of the zero key, suggesting that the zero key be placed below the other keys.

Other studies have explored the effects of button size, button spacing and auditory feedback on number entry speed and accuracy on touch screen devices. Schedlbauer found better performance with larger button sizes [START_REF] Schedlbauer | Effects of key size and spacing on the completion time and accuracy of input tasks on soft keypads using trackball and touch input[END_REF] and Bender et al. [START_REF] Bender | Touch Screen Performance as a Function of the Duration of Auditory Feedback and Target Size[END_REF] found that auditory feedback lasting between 50ms and 400ms only had a significant effect on accuracy for small targets of 10mm x 10mm.

Recently, research in number entry interface design has focused on trying to understand number entry error and improving design to reduce the risk for error. Wiseman et al. [START_REF] Wiseman | A taxonomy of number entry error[END_REF] built a taxonomy of number entry error based on an experiment designed to induce errors while people entered numbers on the numeric keypad. They identified 21 types of number entry errors and organised them into a framework based on the position of their causes in Norman's Action Cycle [START_REF] Norman | The design of everyday things[END_REF].

Oladimeji [START_REF] Oladimeji | Towards safer number entry in interactive medical systems[END_REF] proposes that number entry interfaces can be classified into the following groups: (1.) Serial digit entry describes interfaces that enforce sequential specification of the digits that make up a number typically using a numeric keypad. (2.) Independent digit entry describes interfaces that allow specification of the digits that make up a number in any order. (3.) Incremental number entry describes interfaces that allow number entry through widgets (such as dials, knobs or buttons) that are used to increase or decrease the number.

Thimbleby and Cairns [START_REF] Thimbleby | Reducing number entry errors: solving a widespread, serious problem[END_REF] have shown that the probability of ten-fold errors, which are a significant risk to patient safety [START_REF] Lesar | Tenfold medication dose prescribing errors[END_REF][START_REF] Doherty | Tenfold Medication Errors: 5 Years' Experience at a University-Affiliated Pediatric Hospital[END_REF], can be significantly reduced. They propose a method for parsing the input stream from a serial entry interface so that syntax errors such as multiple decimal points are correctly detected and alerted to the user.

Oladimeji et al. [START_REF] Oladimeji | Number entry interfaces and their effects on error detection[END_REF] compared a serial interface to an incremental interface to explore their effects on error detection. They found that the interface style had a significant effect on the parts of the interface on which users placed visual attention. While the incremental interface encouraged visual attention on the display, the serial interface encouraged visual attention on the input keys. Consequently, the serial interface had more undetected errors than the incremental interface.

Number entry interfaces can often be implemented in a variety of ways. For instance an independent digit entry interface such as that found in Figure 1(d) can be implemented in as many as 28 different ways. Variations in implementation might include whether or not changes to the digits wrap around. This means incrementing the digit '9' turns it to a '0' or decrementing the digit '0' turns it to a '9'. Another variation determines whether the action of increasing or decreasing a digit affects the neighbouring digit to the left. In order words, the implementation performs some arithmetic on the entire number. By running simulated trials of users making keying slips while entering numbers, Cauchi et al. [START_REF] Cauchi | Safer "5-key" number entry user interfaces using Differential Formal Analysis[END_REF] discovered that the differences in the implementation can have effects on the severity of error i.e., by how much an undetected error deviates from the intended number.

With a few exceptions, research in number entry has so far been based on the numeric keypad, usually testing the performance of different layouts. The serial interface offers very quick number entry and its performance scales well as the size of the number to be entered increases. However numbers used for tasks such as infusion therapy in hospitals are from a well-defined range with rules governing the allowed precision of numbers above certain thresholds. For instance, precision of numbers used for rate settings in a critical care unit might be two decimal places for numbers below 10 and only one decimal place for numbers that are between 10 and 100. In addition, from the analysis of interaction logs from infusion pumps used in 4 departments in a hospital, Wiseman et al. found that digits 0, 1, 2 and 5 were the most common digits used when programming infusion pumps [START_REF] Wiseman | Designing for the task: what numbers are really used in hospitals?[END_REF]. These properties have made it feasible to use other number entry interfaces other than the serial interface in the design of medical devices.

For the rest of the paper, we present a detailed description of five interfaces, followed by an analytical evaluation of the speed of the interfaces using the Keystroke Level Model (KLM) [START_REF] Card | The keystroke-level model for user performance time with interactive systems[END_REF] in section 4. We then present details of our user study in section 5, followed by the results and a discussion on the implications of the results. We conclude with some recommendations for number entry interface design.

Number entry interfaces

Based on Oladimeji's classification and implementations found on medical devices, we have implemented five exemplar interfaces: one instance of serial digit entry (number pad), two instances of independent digit entry (up-down and five key) and two instances of incremental entry (chevrons and dial). To evaluate the user interfaces, we built a prototype device with easily customisable keys. Since previous researchers have explored the performance effects of different layout configurations of the serial interface, we evaluate only one instance of the serial interface in our study.

Number pad

This interface allowed number entry using a 12-key numeric keypad in the telephone style layout (see Figure 1(a)). It had a decimal point and a cancel key. The decimal point key appends at most one decimal point to the number on the display. The cancel key deletes the rightmost character on the display.

Chevrons

This interface utilised four buttons in a single row. The two buttons on the left (i.e., the upward facing chevron buttons) increased the number displayed, while the buttons on the right (i.e., the downward facing chevron buttons) decreased the number. Within each pair of buttons, the double chevron buttons caused a change ten times more than the single chevron buttons. This interface allowed two modes of interaction. The user could press the buttons or they could press and hold the buttons. Pressing the buttons changes the displayed number as specified above. Pressing and holding the buttons changes the displayed number at a rate dependent on the duration of hold. Users were expected to press and hold for faster changes to the number.

Up-down

This interface had eight buttons arranged in two rows and four columns. The top row buttons were used to increase the number and the bottom row buttons were used to reduce the number. Each column corresponded to a place value in the resulting number.

For our set up, the rightmost column matched the hundredth place value and was used to increase or decrease the value by 0.01. This interface worked using the arithmetic configuration described by Cauchi et al. [START_REF] Cauchi | Safer "5-key" number entry user interfaces using Differential Formal Analysis[END_REF]. This means the effect of decreasing a digit from 0 or increasing a digit from 9 is carried over to the digit to the left.

Five key

This interface had four1 buttons arranged in a navigation style: up, down, left and right. The left and right buttons moved a cursor on the screen which selected a place value in the number and the up and down buttons increased or decreased the selected digit. Like the up-down interface, it worked using the arithmetic configuration.

Dial

This was a 24-step dial interface with unrestricted continuous rotations in both clockwise and anti-clockwise directions. Users entered numbers on this interface by turning the dial left or right to decrease or increase the number. Quicker turns on the dials caused bigger changes to the number.

Analytical Evaluation

Prior to running our experiment, we analysed the performance of the key based interfaces and estimated task completions times using the Keystroke-Level Model (KLM) for user performance [START_REF] Card | The keystroke-level model for user performance time with interactive systems[END_REF]. This is a model for predicting error free expert performance and as such, we use this prediction as the best-case performance achievable by users of these interfaces. Moreover, we expected that the relative ranking produced by the KLM analyses should be maintained in the results of the experiment.

Numbers used

We obtained log files of 60 syringe pumps from our affiliate University hospital in Swansea. The log files were completely anonymous and contained no personal information. We randomly selected 30 numbers used as rate and volume settings from the logs for our analysis. All the numbers had a decimal part and ranged from 0.26 to 83.3. A third of the numbers used had a precision of 2 decimal places.

Method

Based on simulations of the interfaces used in our experiment, we exhaustively explored the user interface model of each interface using the model discovery technique presented by Thimbleby and Gimblett [START_REF] Gimblett | User Interface Model Discovery: Towards a Generic Approach[END_REF][START_REF] Thimbleby | PressOn: Priciples of Interaction Programming[END_REF]. The user interface model discovery process produces a graph whose nodes represent the states in an interactive system and edges represent the user actions necessary to transition between the states.

To limit the number of states produced by the model discovery process, we limited the numbers addressable by the interfaces to a range covered by those used in the experiment. For each number entered in the experiment, we derived the optimal keying sequence for entering that number on the interface by searching for a shortest path from 0 to N, where N was the intended number. We ran a JavaScript implementation of the A* path finding algorithm, with cost functions that prioritised estimated time of execution over number of button clicks required to enter a number. We estimated the task completion time using standard KLM estimates for pointing and clicking [START_REF] Card | The keystroke-level model for user performance time with interactive systems[END_REF]. As estimated by Card et al. [START_REF] Card | The keystroke-level model for user performance time with interactive systems[END_REF], we used a value of 1100ms for the time (P) taken to point to a button and a value of 200ms for the time (K) taken to click a button. In our prediction of task time, we did not include the time (M) taken for mental preparation because we were interested in the execution time of each task. In our prediction, we do not include the initiation time, i.e., the time elapsed before the task is started or the commit time, i.e., the time taken to click the enter button to confirm the task. Using the keying sequence produced by this process, we derived an approximation for the performance time for executing the sequence.

Given that there are 24 steps in the rotary encoder used in the dial interface, to estimate the time T required to enter a given number N on this interface, we used the following expression: Note that t is the time to perform one-step rotation on the rotary encoder. The value for t was set as 200ms. This is the value (K) taken to click a button.

Result

The Keystroke Level Model analyses produced the estimates displayed in Table 1.

The predictions show that the up-down interface should be fastest with a slight performance edge over the number pad interface and the chevrons interface should be slowest. To validate these predictions, we designed and ran a user study.

Table 1. Approximation of the task times for the different interfaces KLM

Experiment

Design

The experiment was a two-way, mixed design. The within subjects independent variable was the type of number entry interface, and it had five levels: the five interfaces tested. The between subject independent variable was the instruction given to the participant: one group was instructed to enter the numbers as quickly as possible (the speed group) and the second group was instructed to enter the numbers as accurately as possible (the accurate group). We expected the speed group to exhibit more error due to time pressure. The order in which the interfaces were presented to the participants was randomized. The primary dependent variable was the speed of entry of correct numbers. Other dependent variables were the number of incorrect entries, the number of corrected errors.

Participants

There were 33 participants, 17 in the speed condition and 16 in the accurate condition. There were 22 females with 11 in the speed condition. Three participants were lefthanded. The participants ranged in age from 18 -43 with a mean age of 23.5 years (SD=4.86). The participants were undergraduate and postgraduate students in our University. Participants were randomly allocated to conditions.

Prior experience with interfaces. All participants were familiar with the number pad and reported using it on interfaces such as calculators, cash machines and telephones. Five participants (15%) were familiar with variations of the chevrons interface with experience using it in digital stop watches and alarm clocks, eight (24%) had prior completion time using standard KLM estimates for pointing and clicking [START_REF] Card | The keystroke-level model for user performance time with interactive systems[END_REF]. We used a value of 1100ms for the time (P) taken to point to a button and a value of 200ms for the time (K) taken to click a button. In our prediction of task time, we did not include the time (M) taken for mental preparation because we were interested in the execution time of the of each task. In our prediction, we do not include the initiation time, i.e., the time elapsed before the task is started or the commit time, i.e., the time taken to click the enter button to confirm the task.

Using the keying sequence produced by this process, we derived an approximation for the performance time for executing the sequence.

Given that there are 24 steps in the rotary encoder used in the dial interface, to estimate the time T required to enter a given number N on this interface, we used the following expression:

T = ( 10t⇥100 24
+ (N 10)⇥10t 24 if N 10 N ⇥100t 24
otherwise Note that t is the time to perform one step rotation on the rotary encoder. The value for t was set as 200ms. experience with the up-down interface on medical devices and games, nine (27%) had prior experience with the five key interface on remote controls and game controllers and 19 (58%) had prior experience with the dial interface on microwave ovens and temperature controls.

Apparatus

To run our experiments we built a high fidelity prototype unit consisting of a colour display with a resolution of 800 x 480 pixels encased in a box with a configurable front panel and a pole clamp for mounting the device. The front panel served as the input user interface with two variations. The first variation was connected to a 4 x 4 membrane keypad with insert pockets for configuring what is displayed on the keys.

The second variation was a dial powered by a 24 step rotary encoder with tactile feedback on rotation and a selection switch that is activated by pushing the dial. Both front panels were controlled by Arduino boards but with different components attached to them. With these two front panels we were able to configure five types of number entry interfaces; we configured four using the membrane keypad and one using the dial. The different configurations are shown in Figure 1.

We used a pole to mount the prototype unit as shown in Figure 2 and connected the unit to a laptop computer (a 15 inch MacBook Pro). The laptop displayed the instruction for the next trial. Instructions were displayed as numbers in the middle of the laptop screen using a white font color on a black background and a font size of 20px. We used a total of 30 different numbers in the experiment. We used 10 numbers in a practice session and 20 for the experiment. Numbers used were those described in section 4.1. We implemented the software for the experiment in JavaScript and HTML and we logged keystrokes with corresponding time values and the numeric value after each keystroke for all the trials in the experiment.

Procedure

Each study session lasted about 45 minutes and we tested all participants individually. We informed each participant that the experiment involved entering numbers using five different number entry interfaces. Before the experiment started, the participants filled a short pre-experiment questionnaire containing demographic information about their age, gender, handedness and whether or not the participant was dyslexic.

The study itself was in five parts: one for each interface. Each part had a practice session followed by an experiment session. We randomly assigned participants to a speed or accuracy group. We instructed the speed group to enter the numbers in the instruction as quickly as possible and the accurate group to enter the numbers as accurately as possible. We randomised the order in which the users encountered the interface. A laptop computer displayed all study instructions. The instruction was a number displayed in the center of the computer screen. The next instruction was automatically displayed once the participant confirmed entry of the current trial. For the keydriven interfaces using the membrane keypad, participants confirmed entry by pressing a green button on the bottom right corner of the keypad. For the dial interface, participants pushed in the knob to confirm entry. A message signified the end of a session after a participant entered all the numbers required for that session.

Before starting each part of the experiment, the participants watched a video showing them how to use the interface they were about to test. They then had a training session where they tried out using the interface by entering 10 numbers. When they were confident with how the interface worked, they proceeded to the experiment.

The experiment session involved entering 20 numbers using the same interface they used in the training session. These numbers were different from those used in the training session. For each interface tested, each participant entered the same set of 20 numbers, although the order in which the numbers were encountered was randomised. Participants successively entered the numbers displayed in the instruction.

After the experiment, we conducted a short post-experiment semi-structured interview to find out prior experience with the interfaces and the participants relative preference for the interface styles. In return for their time, we gave the participants a gift voucher.

Results

Effect of instruction

Table 2 shows means and standard deviations for each group across the five interfaces. Although we expected more errors in the speed group, an ANOVA showed that group had no statistically significant effect on the participants' speed of entry F(1.67, 51.69) = 0.56, p = 0.55 or accuracy of entry F(2.96, 91.84) = 0.62, p = 0.60. As a result, we combined both groups for the rest of the analysis. We next summarise the results of the speed and accuracy of the interfaces. For the rest of the results below, except the user interface preference statistic, we conducted post-hoc tests using multiple t-tests in order to find out which interfaces differed significantly from the others. For the user interface preference, we conducted post-hoc tests using multiple Wilcoxon Signed-Rank tests.

Speed of number entry

We separated the speed of entry of the interfaces into three constituent parts. The initiation time is the time elapsed between the display of the instruction and the participant's first key press. The execution time is the time elapsed between the participant's first key press and the last key press involved in setting the required number. The commit time is the time elapsed between the last key press in setting the required number and the key press for confirming the task. Initiation time. A one-way repeated measures ANOVA with Greenhouse-Geisser correction found a statistically significant effect of interface style on initiation time F(3.31, 105.85) = 200.08, p < 0.0001. Post-hoc analysis showed that the dial interface had significantly less initiation time than all other interfaces, and the number pad had significantly less initiation time than the chevrons, five key and up-down interfaces.

The dial interface had the shortest initiation time and the chevron interface had the longest. Table 3 shows the mean initiation time for all interfaces.

Execution time. A one-way repeated measures ANOVA with Greenhouse-Geisser correction found a statistically significant effect of interface style on speed of entry F(1.69,54.02) = 425.5, p < 0.001. Post-hoc test showed that the speed of entry of all the interfaces tested were significantly different for all pairs at the 0.001 level. The number pad had the shortest execution time while the chevrons interface had the longest. Table 3 shows the mean execution time for all the interfaces.

Commit time. A one-way repeated measures ANOVA with Greenhouse-Geisser correction found a statistically significant effect of interface style on commit time F(2.84, 90.83) = 24.35, p < 0.0001. Post-hoc analysis showed that the number pad had significantly shorter commit time than all other interfaces and the dial had significantly shorter commit time than the chevrons interface. five key interface had the longest commit time. Table 3 shows the mean commit time for all interfaces.

Errors

We analysed both uncorrected errors and corrected errors. Uncorrected errors were trials for which the user transcribed and confirmed a wrong number whilst corrected errors were keying slips that the user recovered from before confirming the transcribed number.

Uncorrected errors. A one-way repeated measures ANOVA with Greenhouse-Geisser correction found a statistically significant effect of interface style on uncorrected error F (2.98, 95.25) = 4.68, p = 0.004. Post-hoc test showed that the five key interface had significantly less errors than the dial interface t(32)=-3.24, p=0.03. The experiment elicited a total of 57 uncorrected errors, committed by 20 different participants. Only the five key interface was free of uncorrected errors. Table 4 shows the mean uncorrected errors on each interface.

Corrected errors. A one-way repeated measures ANOVA with Greenhouse-Geisser correction found a statistically significant effect of interface style on number of corrections F(2.55, 81.61) = 63.17, p < 0.001. Post-hoc test showed a significant difference for all pairs of interfaces at a 0.01 level, with the exception of the pair updown/five key, which did not differ significantly. Table 4 shows the mean corrected errors on each interface. The experiment elicited a total of 833 corrected errors. The dial interface had the highest number of corrected errors while the number pad had the least.

User interface preference

At the end of the experiment, each user ranked the interfaces in order of preference. We assigned a score of 1 to the lowest preference and a score of 5 for the highest preference. There was a statistically significant difference in the preference rating for the user interfaces χ2 (4) = 73.8, p < 0.0001. The number pad was most preferred interface while the chevrons interface was the least preferred. Table 5 shows the mean ranks for all interfaces. Post-hoc test using multiple Wilcoxon Signed-Ranks test showed that the numeric keypad was preferred to all other interfaces, the up-down was preferred to chevrons, the up-down was preferred to five key, and the dial was preferred to chevrons.

Discussion

Relative preference of interfaces

Since all participants had prior experience using the number pad, it was not surprising that it was rated highest. This preference rating is also reflected in the speed exhibited by the interface. We were however surprised that the dial was not rated significantly worse than up-down and five key interfaces due to the number of corrected errors that occurred on the dial. One possible reason for this could be the significantly shorter initiation time for the dial. In addition, the simplicity of the interface which is based on increasing and decreasing the displayed number means the user has to do little thinking while executing the task. This was articulated by one participant, who said:

Dial was easier to turn the numbers. No need to move your hands from button to button.

Types of errors

The types of errors made during the study spanned across seven classes of errors previously reported in separate studies by Wiseman et al. [START_REF] Wiseman | A taxonomy of number entry error[END_REF] and Oladimeji et al. [START_REF] Oladimeji | Number entry interfaces and their effects on error detection[END_REF].

A summary of all errors is provided in Table 6.

The most common type of error was the Digit Added error. Thirteen different participants made this error on three different interfaces. Oladimeji et al. [START_REF] Oladimeji | Number entry interfaces and their effects on error detection[END_REF] reported this error in their experiment investigating the effect of interface style on error detection. While we classify this error as a member of the Digit Added error type in this paper, we believe the nature of the error makes it different from what the error type suggests. Syntactically, from the numerals that compose the intended number and the transcribed number, the error type suggests that an extra digit has been added to the number. This extra digit, in the case of errors in our experiment, is always zero. Semantically, however, this error appears to involve the inability to correctly understand the difference between the tenths and hundredths part of a number. It is possible that certain people mix up numbers matching the pattern. Indeed one participant transcribed 4.05 for 4.5 and in another trial transcribed 2.5 for 2.05. Over 50% of all unnoticed errors were of this form. Despite featuring on the chevrons, up-down and dial interfaces, this error did not occur on the number pad. This could be because number entry on the number pad is a more direct transcription process of keying a sequence of digits that make up the intended number. Analysis of keystroke logs showed that an instance of this error occurred on the five key interface although it was noticed and corrected.

Difference in speed prediction and study results

We expected the absolute differences in the prediction of results and the actual study results since the participants that took part in the study were not expert users of all the interfaces. As a result, they could not match the optimal performance predicted by KLM. For the numbers used in the experiment, our prediction expected the updown interface to be marginally faster than the number pad. Participants' familiarity with the number pad however meant that their performance was superior on this interface in comparison to the other interfaces. For the number pad, participants actually outperformed the expert model prediction and performed the task in less than half the predicted time. This could be due to the reduced target selection time exhibited in the experiment (554ms) for the number pad in contrast to the standard estimate used in the prediction model (1100ms).

The relative ranking in performance for the interfaces were preserved in the actual experimental data. For the number pad, up-down and five key interface, the observed task completion time for the experiment was less than the predicted time. On the other hand, the observed times for the chevrons and dial interfaces were higher than the predicted time. This difference in prediction could be due to the corrected error rates on the chevrons and dial interfaces which were higher than the corrected error rates With the dial and the chevrons, you don't really know when it switches to higher changes. I tend to stop just before I reach the value I want so that I can increase in one-step changes. For chevrons and dial, really had to time it right and let it go at the right time otherwise could be annoying.

The corrected errors alone do not account for the deviation between the prediction and the recorded performance of the chevrons and dial interface. Another contributing factor is the strategy users develop to reduce the error rates. In the case of the chevrons interface, users employed discrete click interactions rather than the faster press and hold interaction, and for the dial, users made slower but more careful turns. Figure 3 shows that the cost of correcting an error is larger on both these interfaces in comparison to the others.

Effects of interface style on number perception

The different styles of interfaces had an effect on how numbers were perceived by the users. When users entered numbers on the serial interface (the number pad) or the independent digit interface (up-down or five key), they were more likely to think about the numbers as a sequence of digits without thinking much about the numeric quantity of the number as a whole. Whereas, using an incremental interface such as the chevrons or dial, participants were more likely to concentrate more on the number as a whole. One participant commented that:

For the number pad, up-down and the five key, I did not think of the number as a whole, just entered them digit by digit but for the chevron and dial, I had to understand the number.

7.5

Severity of errors committed

The types of errors committed were closely related to the interface used to enter the number and consequently the severity of error, i.e., the deviation of the intended number from the transcribed number or the ratio between the intended and the transcribed number. Theoretically, the number pad and the up-down interfaces have the potential for producing the largest deviations from the intended number based on keying slips. This is due to the possibility of missing decimal points and missing digits on the number pad and the possibility of wrong place value on the up-down interface. We defined three levels of error severity based on the errors committed in our experiment. Low error severity referred to those errors where the ratio between the intended number and the transcribed number is at most 2, medium error severity refers to when the ratio is at most 10 and high error severity refers to when the ratio is greater than 10. Table 7 shows a summary of all errors committed and their severity.

Incremental interfaces and varying number precision

As is typical of setting up some infusion devices used in hospital critical care, the set of numbers used for the study required that numbers below 10 were precise to two decimal places while numbers from 10 and above were precise to one decimal place. This factor meant that the display of incremental interfaces would only render numbers to the appropriate precision. As a result of this, button functions changed modes when the precision of numbers change on the display. For instance on the chevrons interface, when users change the value 9.99 to 10.0, the double chevron button changes meaning from 'increase by a tenth' to 'increase by a unit'. Similarly on the dial interface, one turn on the dial changes meaning from 'increase by a hundredth' to 'increase by a tenth'. We based our implementation of the chevrons interface on a medical device. It was also evident that some participant found the hold-down mode of the chevrons very difficult and challenging to predict. In this mode, the longer the buttons were held down, the larger the increments made to the number. This mode change was confusing for some users. Two participants expressed that:

For chevrons, the increments were very confusing. The same button did two jobs and the mode changes are confusing. Sudden changes were very confusing . . . for example you could go from 30 -60 in a very short time span and then going back restarts the counter and climbs up rapidly. . . Chevrons, seem to jump quite a lot, took too long to get to intended number. Same problem with dial. It goes in sequential order rather than control individual digits.

The feature of varying precision described in this section is a requirement in infusion pumps used in critical care and intensive therapy units where low dose settings are common. It remains a design challenge to create an incremental interface that supports this form of varying precision in a way that is not confusing to the user.

Conclusions

Number entry is ubiquitous and number entry interfaces are very common in interactive devices. Unlike text entry, it is difficult to build a predictive model that suggests corrections for number entry error because a number entry error is usually more ambiguous than text entry errors.

In order to approach more dependable design for number entry interfaces, particularly those in use in safety critical scenarios like in medical devices, we have explored the performance differences between five different interfaces. Our results show that the number pad is the fastest and five key is the most accurate. With the results of our study, we make suggestions to designers concerning the trade offs to expect when choosing between different styles of number entry interfaces as well as the likely errors on an interface style.

The number pad offers the fastest mode for number entry but it comes with the risk of high severity errors such as tenfold errors. These high severity errors are mostly caused by unintentional repeated digits or unintentional missing digits. Designers should guard against repeated digits caused by overly sensitive keys, e.g., those caused by key-bounce errors [START_REF]ISMP: Double key bounce and double keying errors[END_REF]. Repeated decimal points should be properly parsed and alerted to the user as an error, as suggested by Thimbleby and Cairns [START_REF] Thimbleby | Reducing number entry errors: solving a widespread, serious problem[END_REF]. Keypad hardware should be rigorously tested to guard against missing digits which might be caused by keys that provide tactile feedback even when they have not been completely activated electronically.

Incremental interfaces, like the dial or chevrons, have the advantage that users place their visual attention on the display of the interface so they are more likely to notice and correct any errors. They are however much slower for entering numbers and can be frustrating to users due to high likelihood of overshooting and undershooting the target value. Designers should explicitly indicate to the user, what place value in the number is being edited by using a cursor. Designers should consider offering a more direct control of the rate of change of values in order to reduce the error rate and consequently, task time for these interfaces.

For the up-down designers should be aware that numbers might be shifted to a wrong place value e.g., 24.5 might be transcribed as 2.45 since the decimal point is not explicitly set and the number entry keys might not be perfectly aligned to the digits they affect. Designers should consider labeling the keys on this interface or using this interface style on touch screens where input and output are on the same media and the cost of changing key layout is minimal.

Prior to this study, the majority of studies on number entry interfaces have been limited to the numeric keypad, mainly studying the effects of key layout on performance, both speed and accuracy. Our study covers a wider design space for number entry interfaces. The results provide information to designers about the tradeoffs involved in choosing one interface style over the other.

Fig. 1 .

 1 Fig. 1. The different configurations of interfaces used in our setup. From left to right (a) Number pad (b) Chevrons (c) Up-down (d) Five key (e) Dial

Fig. 2 .

 2 Fig. 2. The setup for the experiment showing the prototype mounted on a pole

Fig. 3 .

 3 Fig. 3. A comparison of the actual and predicted performance for each interface

Table 2 .

 2 Mean and standard deviation for the speed and accuracy of entry between the groups

			Entry Accuracy			Entry Speed
		Speed Group	Accurate Group	Speed Group	Accurate Group
		Mean	SD	Mean	SD	Mean	SD	Mean	SD
	Number pad	0.29 0.77	0.06	0.25	1906	423	2266	466
	Chevrons	0.65 1.17	0.19	0.54 13355	3122 14471	2691
	Up-down	0.65 1.69	0.38	0.62	3990	745	4783	1210
	Five key	0	0	0	0	5231	908	5911	1213
	Dial	0.94 1.92	0.44	0.81	9072	1211 10276	2024

Table 3 .

 3 Mean and standard deviation for the initiation, execution and commit times

		Initiation	Execution	Commit		Total
		Mean	SD	Mean	SD	Mean	SD	Mean	SD
	Number pad	1286	339	2080	474	730	223	4096	821
	Chevrons	1535	433 13896	2931	1005	291	16436	3231
	Up-down	1469	392	4374	1061	970	323	6813	1531
	Five key	1463	354	5561	1105	1017	389	8040	1571
	Dial	167	51	9655	1740	910	282	10732	1861

Table 4 .

 4 Mean and standard deviation for the corrected and uncorrected errors.

		Corrected Errors		Uncorrected errors
		Mean	SD	Mean	SD
	Number pad	0.88	1.08	0.18	0.58
	Chevrons	6.48	3.86	0.42	0.94
	Up down	2.73	2.74	0.52	1.28
	Five key	3.09	2.38	0	0
	Dial	12.06	5.53	0.70	1.24

Table 5 .

 5 Mean ranks for interface preference.

		Number pad Chevrons	Up-down	Five key	Dial
	Mean Rank	4.81	1.69	3.50	2.44	2.56

Table 6 .

 6 Frequency of errors made during the experiment.

	Error type	Total	Interfaces	Example
	Digit added		31 chevrons, up-down, dial	4.05 for 4.5
	Wrong digit		8 chevrons, up-down, dial	60.5 for 62.5
	Missing decimal		3 number pad	249 for 24.9
	Out by ten		3 number pad, up-down	1.11 for 11.1
	Missing digit		1 number pad	6.5 for 62.5
	Skipped		4 number pad, chevrons, up-down,	'' for 62.5
	No clear reason		dial 9 chevrons, up-down, dial	56.7 for 3

Table 7 .

 7 The severity of undetected errors committed on each interface.

		Total Errors		Error Severity
			Low	Medium	High
	Number pad	4	0	3	1
	Chevrons	11	10	1	0
	Up-down	16	13	3	0
	Five key	0	0	0	0
	Dial	21	16	5	0

Although the reader might see this as a four key interface, we refer to this interface as five key to be consistent with naming conventions in literature, for example see Cauchi et al.[START_REF] Cauchi | Safer "5-key" number entry user interfaces using Differential Formal Analysis[END_REF]

Acknowledgements. Funded as part of CHI+MED: Multidisciplinary Computer-Human Interaction research for design and safe use of interactive medical devices project EPSRC Grant Number EP/G059063/1. Carlos Monroy Aceves played a key role in the design of the prototype device.


Christiane Wölfel
email: woelfel@cip.ifi.lmu.de

Timothy Merritt
email: timothy.merritt@aarch.dk

Method card design dimensions: a survey of card-based design tools

Keywords: method cards, creativity cards, design methods, design tools

There are many examples of cards used to assist or provide structure to the design process, yet there has not been a thorough articulation of the strengths and weaknesses of the various examples. We review eighteen card-based design tools in order to understand how they might benefit designers. The card-based tools are explained in terms of five design dimensions including the intended purpose and scope of use, duration of use, methodology, customization, and formal/material qualities. Our analysis suggests three design patterns or archetypes for existing card-based design method tools and highlights unexplored areas in the design space. The paper concludes with recommendations for the future development of card-based methods for the field of interaction design.

Introduction

'Design methods are like toothbrushes. Everyone uses them, but no one likes to use someone else's.' [START_REF] Harrison | On Methods[END_REF] Physical cards have been popular design tools, perhaps because they are simple, tangible and easy to manipulate. Aside from the well-known Card Sorting method [START_REF] Hudson | Playing your cards right: getting the most from card sorting for navigation design[END_REF], cards have been used widely by designers to make the design process visible and less abstract [START_REF] Lucero | PLEX cards: a source of inspiration when designing for playfulness[END_REF][START_REF] Lafrenière | Variations of a theme: card-based techniques for participatory analysis and design[END_REF] and serve as communication tools between members of the design team and users [START_REF] Beck | Instant card technique: how and why to apply in user-centered design[END_REF]. There are many examples of unique method card systems, many have similar features and formal qualities, yet it is not easy to get an overview of the available card systems in order to decide which to use, and when. As suggested in the opening quote attributed to John Zimmerman, designers often develop their own methods or appropriate widely known methods to best suit their needs, yet there is a tendency to use methods that are familiar instead of venturing out. Even though these methods are shared with the research community, it is difficult and time consuming for designers to review all available tools to understand their strengths and weaknesses. Therefore, this paper aims at providing an overview of some well-known sets of method cards including features and limitations so that the interaction designer can quickly review and make informed choices when selecting card-based method tools or to serve as a source of inspiration as they develop and appropriate a suite of design methods of their own. We begin by analyzing different card systems according to five design dimensions, which suggest possible archetypes or patterns for these tools. We then discuss limitations and future work on card-based design tools that could benefit interaction design researchers.

Analysis of card-based design tools

There are many examples of cards being used to assist or provide structure to the design. We selected and examined eighteen card systems through direct use and reviewed the accompanying literature to gain an understanding of the authors' intentions for use. As first steps toward describing the various card-based systems, we identified five design dimensions. While this is not an exhaustive survey, it begins to provide a sketch of the card-based method landscape and highlights key differences among the attributes of many card-based tools. This approach has been used to articulate the design space of various examples of interactive media [START_REF] Cheok | Empathetic living media[END_REF][START_REF] Pousman | A taxonomy of ambient information systems: four patterns of design[END_REF], and is used in the present paper to encourage the design community to engage and develop the field of card-based tools. When examined according to these dimensions, three broad groups of card systems emerge suggesting possible archetypes or design patterns [START_REF] Alexander | A Pattern Language: Towns, Buildings, Construction[END_REF]. A secondary contribution is that our review assists the review and selection of cards for design researchers and practitioners.

Design Dimensions

As stated previously, our aims in defining the five dimensions-and graduations within the dimensions-are in revealing key differences across the examples including: 1) Intended Purpose & Scope, 2) Duration of use and placement in design process, 3) System or Methodology of use, 4) Customization, and 5) Formal Qualities. These attributes describe claims from the literature of the authors, the formal characteristics, and the tools in use. While these may seem closely related, it is an initial step in developing a framework for discussing the design attributes in card-based tools. Graduations within these dimensions were chosen to differentiate the examples-future work is needed, however to validate and develop these further.

Intended purpose & Scope.

Based on research literature or from booklets and inserts included in the card packages, the respective authors have made claims as to where their tools fit within the design process (ideation, inspiration, engaging non-designers, etc.). In this category we can ask: where in the design process are the cards used and how should they be used? Do they have a specific purpose and do they focus on a particular context? We identified three graduations of intended purpose & scope, ranging from very general to context specific.

General/Repository card systems provide inspiration and challenge designers to take another point of view. An example here are the Oblique Cards [START_REF] Eno | Oblique strategies[END_REF][START_REF] Burns | Oblique Strategies for Ambient Journalism[END_REF], which can be engaged with at any time in any context to increase lateral thinking and stimulate design problem solving in general. These types of cards aim for open-ended inspiration with little or no guidance on their use. These cards mainly function as repositories for design methods, capturing well-known methods from important literature [START_REF] Rogers | Interaction Design: Beyond Human -Computer Interaction, 3rd Edition[END_REF] and offload the task of remembering the many design methods.

We also found various examples of cards, which focus on participatory design. They seek to develop sensitivity and empathy [START_REF] Bowen | Grounded theory and sensitizing concepts[END_REF] for the context, and engage designers and users in the process. Some cards are designed for a better communication between users and designers, examples here are the Questionable Concept Cards [START_REF] Vines | Questionable concepts: critique as resource for designing with eighty somethings[END_REF], which encourage criticism and debate or the Inspiration Cards [START_REF] Halskov | Inspiration card workshops[END_REF] that require collaborative work between designers and domain experts using the cards.

There are also context specific/ agenda-driven examples. This includes those cards focused on a particular context or design agenda as the Sound Design Deck [START_REF] Alves | A deck for sound design in games: enhancements based on a design exercise[END_REF], which facilitates sound design in games or the Design Play Cards [START_REF]Eco Innovators: Design Play Cards: Designing for Sustainability[END_REF], which focus on designing for sustainability.

Duration of use/ when in process.

It is important to acknowledge the time investment that the various systems requireand to know when in the process they are used. This dimension includes key differences in the length of time ranging from one time use to sustained use of the system throughout the design process. Another aspect is the placement in the design processwhether the cards should be used in the very beginning, after initial field studies or prior to mockup sessions and prototyping. Four groups were identified, which range from anywhere/anytime to at a specific point in time.

The Oblique Cards are an example of cards that can be used anywhere/any time in the process. They can be useful in the very first phase of idea generation, but also when facing problems during the design, being stuck or looking for alternatives. Cards presenting a collection of methods as the IDEO Method Cards [START_REF]IDEO: IDEO Method Cards: 51 Ways to Inspire Design[END_REF], are often positioned to be used as needed. As they provide a lot of different methods, some of those will fit in an early design stage, whereas others are for evaluation and testing. Other cards should be used at the beginning of the process as they provide input for further concept development; for example, PictureCARDs [START_REF] Tschudy | PictureCARD: A Storytelling Tool for Task Analysis[END_REF] are used after an initial field study and provide the basis for the card creation.

The last aspect of time is that cards are used at a specific point, for example in a workshop. The Sound Design Deck is used in this way, when applying the introduced methodology. But even though most of the work with the cards is done in a short session (~2h), one should still refer back to the cards later in the design process. Methodology of use. Some of the cards can be used very freely, whereas others provide a methodology how to use the cards. Some of the approaches are playful and game-like; some have rules or discreet steps that should be followed. This can be helpful to get started using the cards but might at the same time be restrictive. We identified three groups in this category: no methodology, suggestion for use and specific instructions.

Cards with no system are used ad-hoc with no suggested structured process provided by the authors. Cards of this type include IDEO [START_REF]IDEO: IDEO Method Cards: 51 Ways to Inspire Design[END_REF], SUTD [14], and Oblique [START_REF] Eno | Oblique strategies[END_REF]. Most of the cards offer at least a basic suggestion for use. The DSKD Cards [START_REF]DSKD Method Cards[END_REF] come with a small brochure, which has some examples how the cards can be used. The authors of the PictureCARDs describe how they were using the cards, but there are no hard and fast, specific rules. The last category describes cards in which specific instructions are given. The authors of the Sound Design Cards introduced a specific method of how to use the cards, including a workspace with four regions in which cards can be moved, thus facilitating idea generation and keeping track of the design work at the same time. Inspiration Cards [START_REF] Halskov | Inspiration card workshops[END_REF] also provide specific instructions, noting where the cards should be arranged on a poster to formulate a design idea.

Customization. Although we acknowledge that any technology tool will be adapted and appropriated into the user's life, in this dimension, we describe the degree to which the tool provides for customization as part of its use. The first group we identified in this category is no customization. When we examine the SUTD, Oblique, etc cards, they are intended to be static and unchanged. Cards offering trivial customization, do not allow the user to add or modify content, but only to structure or group the cards. This is the case with the IDEO iPhone app that in most respects replicates the paper cards [START_REF]IDEO: IDEO Method Cards: 51 Ways to Inspire Design[END_REF] allowing the user to make groups and add cards to the groups. The Sound Design Deck provides for optional customization, whereas users can create their own cards and add them to a wiki. This is intended and welcome by the authors, as they aim to create a pattern language for sound design. The last group of cards requires customization in order to be utilized. Examples here are the Inspiration Cards, the Ideation Deck [START_REF] Golembewski | Ideation decks: a card-based design ideation tool[END_REF] or Questionable Concept Cards. The cards have to be created beforehand and are therefore applicable in the specific project, which helps the designer to get a better understanding of the project domain.

Formal qualities. While the focus of this paper is on "cards", there are differences in the physical properties (2-sides, paper, size, shape), connections to virtual systems (stand alone or connected to objects in the room or in the virtual world), and appearances (images, diagrams, words, color schemes, etc.) Other formal qualities include issues such as the fact that some card systems have only one copy of a card vs. multiple copies, etc. We do not provide for all possible configurations, however, we provide graduations according to the use of media. The simplest type of cards have only text or only images, while most of the cards combine text and image or illustration, like the Inspiration Cards or the PLEX Cards [START_REF] Lucero | PLEX cards: a source of inspiration when designing for playfulness[END_REF]. The authors of PLEX Cards present their evaluation of the cards, and describe feedback regarding the images. This feedback highlights the importance of choosing suitable images for cards-they claim that the image should be abstract enough to allow an open interpretation, but at the same time detailed enough so that the user can relate to and interpret it. There are various card systems where the content is divided into different categories, as with the IDEO Method cards or the SUTD Cards, which provides thematic structure in the cards and suggests how the cards relate to each other. Finally, there are some cards, which have a virtual component, as in the Sound Design Deck, which connects the physical cards to the online wiki providing additional information and example videos.

Fig. 1. Classification of method cards

Toward Patterns of Design

By plotting the card systems as shown in Figure 1, there are three archetypes or patterns of design [START_REF] Pousman | A taxonomy of ambient information systems: four patterns of design[END_REF][START_REF] Alexander | A Pattern Language: Towns, Buildings, Construction[END_REF] that emerge: general purpose/repository cards, customizable cards, and context specific cards. Further analysis and refinement of these groupings will be taken up in future work, however we can provide initial comments. Cards that are classified as "general purpose/repository" cards offer either a method repository or aim to stimulate inspiration and lateral thinking. They can be used during the whole process and only have general or no instruction for use. In most cases the deck is split up into different categories yet used without customization. "Customizable" cards are inherently customizable to some degree. Interestingly, we can see that most of these belong to the participatory design group and have specific instructions on PictureCARD (PC) [START_REF] Tschudy | PictureCARD: A Storytelling Tool for Task Analysis[END_REF] Sound Design Deck (SDD) [START_REF] Alves | A deck for sound design in games: enhancements based on a design exercise[END_REF] SUTD [14] Layered CARD (LC) [START_REF] Muller | Layered participatory analysis: new developments in the CARD technique[END_REF] Questionable Concept Cards (QCC) [START_REF] Vines | Questionable concepts: critique as resource for designing with eighty somethings[END_REF] IDEO [START_REF]IDEO: IDEO Method Cards: 51 Ways to Inspire Design[END_REF] Inspiration Cards (IC) [START_REF] Halskov | Inspiration card workshops[END_REF] Instant Card Technique (ICT) [START_REF] Beck | Instant card technique: how and why to apply in user-centered design[END_REF] SILK [START_REF] Silk | Method Deck[END_REF] Design Heuristics (DH) [START_REF] Daly | Assessing Design Heuristics for Idea Generation in an Introductory Engineering Course[END_REF] Eco Innovators' Cards (EIC) [START_REF]Eco Innovators: Design Play Cards: Designing for Sustainability[END_REF] DSKD [START_REF]DSKD Method Cards[END_REF] Ideation Deck (ID) [START_REF] Golembewski | Ideation decks: a card-based design ideation tool[END_REF] Oblique Strategies (OS) [START_REF] Eno | Oblique strategies[END_REF] Bootleg (BL) [25] PLEX Cards (PX) [START_REF] Lucero | PLEX cards: a source of inspiration when designing for playfulness[END_REF] Visual Explorer Cards (VEC) [START_REF]Center for Creative Leadership: Visual Explorer Post Card Facilitator's Set[END_REF] MethodKit(MK) [START_REF] Möller | MethodKit. Idea Society[END_REF] Purpose&Scope how to use them to engage with end-users and non-designers. They are mainly used at a specific point in the design process and are composed of text and images. The third group includes "context specific" cards that are developed primarily to focus on a specific design agenda or context. They are primarily used at a specific point in time with specific instructions. In the next section we discuss how this exercise of analysis helps the design research community make sense of card-based tools.

Discussion

The classification exercise and resulting design patterns raise important issues for designers about when the cards can be used in the design process and what one should consider when choosing a card deck. We propose future work enabling customization and extending physical cards into digital media or augmented representations. To conclude, limitations of the present paper are provided.

Using and choosing design method cards

As noted earlier, designers can choose to use method cards in their design process either by using existing systems or by creating a new set of cards-essentially, should the designer use an existing toothbrush [START_REF] Harrison | On Methods[END_REF] or create a new one? This decision depends on several factors including time, knowledge of team members, needs of the design team, expected results and many other possible factors. Through the process of reviewing and analyzing the card systems and accompanying literature it would seem beneficial for designers to become familiar with at least one example of each of the three card archetypes we identified before choosing or creating a card-based tool.

After reviewing these, the designer should evaluate the time and resource investment that can be made and to stimulate ideas about possible formats for a card system if they determine that they will build a tool to be used on an ongoing basis. If a new card system is created, perhaps the most critical attribute is the degree of customization that is expected-whether the cards are living and evolving, or serve more as a lasting resource of commonly needed methods. In the next section we engage with the issue of customization of card systems and the use of technology as ways to extend traditional card-based tools.

Customization and use of new technology

It was surprising that only few examples were found that offer customization directly, or in some form of digital component to augment the tool. There is much interest in the development of robust technologies including QR codes, augmented reality, e-ink, etc. that bridge the gap between the digital and physical world with the aim of engaging people into complicated work processes [START_REF] Zhao | Magic cards: a paper tag interface for implicit robot control[END_REF] or to capture lasting impressions of work. Showing similarities between individual cards within different card systems could go much further to reduce the time spent in the design process searching for appropriate methods. Various digital technologies could facilitate this by showing connections virtually or presenting additional information augmented onto the cards on the table or wall. Designers could add their own notes to cards and share those with their team members, which was a future development suggested also in [START_REF] Halskov | Inspiration card workshops[END_REF], yet robust examples were not found in any of the card system except in [START_REF] Alves | A deck for sound design in games: enhancements based on a design exercise[END_REF], however, it is not clear that the digital components are used for anything more than a repository. This does however go beyond the digital representation approach offered in the iPhone App for the IDEO Method Cards. There is no connection to the physical world except that all of the digital cards are copies of the physical cards-there is no customization other than adding cards to groups. In a sense, the digital app loses functionality-the designer can not look at more than one card at a time, can not write on the card itself, and there is no new information despite the content being more than ten years old. In our future work we intend to explore meaningful ways of connecting physical card-based tools with interactive digital elements.

Limitations

While the present paper begins to sketch out the design space for design method cards, there are limitations that we would like to acknowledge. Our list of method cards does not represent all method card systems, yet eighteen examples is adequate to begin to understand the design space. We also did not provide in-depth reviews of each of the card systems, excluded the number of cards in each deck, etc. These limitations do not diminish the contribution here, but rather signal important future work.

Fig. 2 .

 2 Fig. 2. Method card design patterns


Joanna Hare
email: jhare@pdronline.co.uk

Steve Gill
email: sjgill@cardiffmet.ac.uk

Gareth Loudon
email: gloudon@cardiffmet.ac.uk

Alan Lewis
email: profalan.lewis@wales.ac.uk

The Effect of Physicality on Low Fidelity Interactive Prototyping for Design Practice

Keywords: Physicality, interactive prototypes, computer embedded products, design, product design, iterative product development, information appliances

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

This paper builds on previous research on physicality and low fidelity interactive prototypes. Virzi et al. [START_REF] Virzi | Usability problem identification 2 Costs are those shown in Figure 4 minus the software prototyping (shared by all prototypes). using both low and high-fidelity prototypes[END_REF] found that there was little difference in usability data for high and low fidelity models of standard two dimensional graphical interfaces and an interactive voice response system. Yet a nu mber of researchers [START_REF] Lim | The Anatomy of Prototypes: Prototypes as Filters, Prototypes as Manifestations of Design Ideas[END_REF] [START_REF] Mccurdy | Breaking the fidelity barrier: an examination of our current characterizat ion of prototypes and an example of a mixed -fidelity success[END_REF] felt that the concept of lo w verses high fidelity is not quite enough to convey the whole manner of situations that prototypes are constructed for. McCurdy et al. [START_REF] Mccurdy | Breaking the fidelity barrier: an examination of our current characterizat ion of prototypes and an example of a mixed -fidelity success[END_REF] argued for a mixed approach that allowed various aspects of a prototype to be built at different fidelity levels according to the design component being prototyped. They go on to suggest that there are five 'dimensions' or fidelity aspects that can be defined as somewhere between high and low within the same prototype, namely, aesthetics, depth of fun ctionality, b readth of functionality, richness of data and richness of interactiv ity. So far this concept of mixed fidelity has been trialled with software but not physical prot otypes. Despite several authors conducting studies on prototypes of computer embe d-ded devices the physical properties of both the model and interaction have been larg ely ignored.

In 2008 we demonstrated that in order to trial an interactive device with users an interactive prototype must be constructed [START_REF] Gill | Rapid Develop ment of Tangib le Interactive A ppliances: Achieving the Fidelity / Time balance[END_REF]. The same study went on to lower both the level of physical fidelity of the model and the visual fidelity of interface until usability data started to significantly differ fro m the results of the final device. It was proposed that subtle differences in physicality, in this case removing the tactile feedback of buttons, affected the results suggesting that considerations of physicality are more important than the level of fidelity. This poses the question of how we 'consider' physicality.

Later work however demonstrated that some effects of physicality on user trials were only apparent through in-depth analysis because the effects were often subtle and the picture sometimes confusing [START_REF] Hare | Physical Fidelity: Explo ring the Importance of Physicality on Physical-Digital Conceptual Prototyping[END_REF]. This study seeks to clarify the position physicality occupies in user interactions .

The 2009 study sought to uncover the resulting differences in physicality based on low, mediu m and high(er) fidelity prototypes. In this study physicality was considered to fall under two areas: the physicality of the device (e.g. form, fin ish, weight) and the physicality of the interaction (the feel of the buttons and wheel in this case). But this method only allows the prototypes to be described and not directly compared which is essential when using physicality to determine the differences between the prototypes on trial. The physicality of the device and interaction was an appropriate way to describe the prototypes and, with subsequent analysis, this has been adapted to form the concept of 'passive' and 'active' physicality where:

Passive Physicality is how the prototype looks and feels when turned off, for e xamp le the weight, finish and button locations.

Acti ve Physicality is how the prototype reacts to the users, typically the reaction of the interface (software), the feel o f the buttons when operated (or sliders, dials, screen etc.)

To explain these terms a useful starting point is that of Dix et al. [START_REF] Dix | Physigrams: Modelling Devices for Natural Interaction[END_REF] who regard the physical device removed fro m its context and 'separated' fro m its dig ital operation in order to consider the mapping of the device 'unplugged'. Th is is the basis of 'passive' physicality; the judgments that can be made about the device with out switching it on. Do you grasp a cup by its handle or by the body? Decisions are made about the co mfort of the cup's handle by its appearance and the perceived weight of the contents of the cup [START_REF] Reeves | Physicality, spatial configuration and computational objects[END_REF]. Passive physicality also has its roots in Gibson's description o f affordances [START_REF] Gibson | The theory of affordances[END_REF] wh ich suggest ways of interaction. Affordances are not simply a property of the object; they are the way a specific user relates to that object. When Norman [START_REF] Norman | The Design of Everyday Things. Basic Books[END_REF] applied Gibson's idea to design; he divided the idea of affordances into those of real and perceived affordances. Whilst real affordances tell the user what they could actually do with the device, meaningful or not, perceived affordances tell the user 'what a ctions can be performed on an object and, to some extent, how to do them'. Yet pa ssive physicality is more than affo rdances, it includes the phys ical properties of the device, its weight, finish and locations of the interactions.

Active physicality is concerned with the interactive port ion of the device; what happens when the device is being used. It is still the physical that is o f concern but in relation to the device's purpose and ease of use; how buttons operate the interface and how those buttons (or any interactions) feel when operated.

The exact drivers behind active and passive physicality might differ depending on the product being prototyped but the essence of active and passive physicality will remain.

This study proposes that a prototype can be considered by its level of act ive and passive physicality. For examp le, a prototype that is driven by the technology of the experience rather than the proposed size o f the design would have a high level of active physicality but low passive physicality.

By attempting to understand physicality and using this to drive the physicality of low fidelity prototypes we aim to draw out just how physicality can be used by the designer to create efficient low fidelity prototypes. The efficiency of a prototype is of great importance; an efficient prototype can supply reliable data for a fraction of the cost of a high fidelity prototype enabling an iterative process . The early stages of the typical user-centred design process are h ighly iterative in order to react to and inform the developing project. User t rials are a key tool to gathering data needed to inform the project, techniques include rapid ethnography [START_REF]The Invisib le Co mputer: Why good products can fail, the Personal Co mputer is so complex, and informat ion appliances are the answer[END_REF], usability evaluation [START_REF] Nielsen | Usability Engineering[END_REF] and task centered walkthroughs all of which can be supported by interactive prototypes, and these prototypes need to be fast, low-cost and stage appropriate. This paper presents an early stage study on four low fidelity prototypes of the same device.

Methodology

The prototypes

An existing product was chosen to provide a datum against which the retrospectively developed prototypes could be measured. The choice to retro -prototype an existing device as a method was taken after considerable thought. The alternative would have been the development of a new device. Both methods have been used in prot otype evaluation studies [START_REF] Gill | Rapid Develop ment of Tangib le Interactive A ppliances: Achieving the Fidelity / Time balance[END_REF] [START_REF] Lim | Co mparat ive Analysis of High-and Low-fidelity Prototypes for More Valid Usability Evaluations of Mobile Devices[END_REF]. Retro-prototyping was chosen because it has the benefit of access to a real, mass produced product, identified by the manufacturer as a worthwhile idea and having successfully undergone a product development process. The finished device can be used to compare the results from the user study in a manner that is all but impossible to recreate in a research study.

Fig. 1. The iRiver SPINN

The product chosen was the iRiver Sp inn (Figure 1), a personal music player. The main features and interactions of the iRiver Spinn are shown in Figure 2.

Four low fidelity prototypes were constructed using techniques currently in use in industry. Each prototype was planned giving due consideration to active and passive physicality levels, with the intention of placing one in each of the quadrants shown in the graph in Figure 3.

Fig. 2. The interactions of the iRiver Spinn

Fig. 3. Areas of physicality

The time taken to make each type of prototype is a critical issue. 'Time is money' and so we timed the building process and applied an hourly rate of £40 in order to cost each prototype. These are shown in Figure 4.

Fig. 4. The time taken to construct the prototypes

A single interface core was coded in Adobe Flash for all prototypes and adapted to the needs of each. Preparatory work ensured that this interface would be suitable for all p rototypes and that the adaptation of the interface was possible for all. As is typ ical at this stage of the design process, only a limited selection of features were inclu ded in the software [START_REF] Nielsen | Usability Engineering[END_REF]. A single Co mputer A ided Design (CAD) model was created.

Prototype 1 (Figure 5; named 'blue foam') was constructed from model making foam board. Interaction was based on the Wizard of Oz technique [START_REF] Maudsley | Prototyping an intelligent agent through Wizard of Oz[END_REF], the Flash interface was operated remotely by the facilitator and viewed on the Tablet, the participant was asked to follow the 'think out loud' protocol [START_REF] Gou Ld | Designing for Usability -Key Princip les and what Designers Think[END_REF], the facilitator could react to what the participant was saying and interacting with on the foam prototype. The physical model for Prototype 2 (Figure 6; named 'IE4') was constructed using rapid prototyping techniques (FDM). The CAD model was adjusted slightly to house the buttons and the dial wh ich were integrated to make the prototype interactive. An IE4 [START_REF] Gill | IN PRESS: PAIPR Prototyping: So me Thoughts on the Role o f the Prototype in the 21st Century[END_REF] 1 was used to connect the buttons to a laptop. The Flash interface, shown on a tablet, 'listens' for key presses fro m the IE4 and triggers changes in the interface when the participant interacts with the prototype.

Fig. 6. Prototype 2: IE4

The physical model for Prototype 3 (Figure 7; named 'appearance model') was intended to reflect the final device as accurately as possible. The form was rapid prototyped (using FDM) then finished to facsimile level. The Flash interface was operated by the participant on a touch screen tablet.

Fig. 7. Prototype 3: Appearance prototype

A rough foam model was constructed for Prototype 4 (Figure 8; named 'Arduino') to accommodate the off-the-shelf buttons and dial. The dial was connected to an Arduino [START_REF] Burleson | Sprock-it : a physically interactive p lay system[END_REF] wh ich received the analogue signals and outputted them to the co mputer running the Flash interface. The buttons were connected to an IE4. Due to the ext ra code required for the Arduino, the interface was shown on a lap top rather than the touch screen tablet.

Assessing Physicality

Each of the prototypes was analyzed in terms o f active and passive physicality. The main factors in the design that would determine the passive physicality levels of the prototype were determined to be: scale, form, finish and button location. For active physicality the main issues were: Spinn physical feedback, Sp inn digital feedback, button physical feedback and button digital feedback. Initially a 'scoring' system was trialed but this was discarded, for when we call a prototype 'low' fidelity we do not assign that 'lowness' a value, as designers we intrinsically know when a prototype is low fidelity. It is only when conducting studies such as this that a prototype is considered lower or h igher than another. Figure 9 shows the considerations for assessing each prototype.

Prototype

Passive

Appearance model

High

The prototype looks and feels very similar to the final product.

Low

The interactions are not obvious as the participant does not use the tangible prototype to operate the interface; instead the interface is operated on a touch screen breaking the link between the tangible product and its interface.

Arduino Low

The prototype has tacked on switches and wires are distractingly apparent in both the aesthetics and tangibility of this prototype.

High

The prototype accurately mimics the way the final device feels when it is operated, both in the way the buttons work and the functionality of the interface.

Fig. 9. Assessing the levels of active and passive physicality of the prototypes

The Appearance and Arduino prototypes are high in one area of physicality at the expense of the other, whilst the Foam and IE4 prototypes 'balance' both active and passive physicality, as shown in Figure 10.

2.3

The User Study 40 part icipants were recruited for the study (eight per prototype [START_REF] Goodwin | Designing for the Dig ital Age: Ho w to create humancentered Products and Services[END_REF]), two did not turn up and three tests were rejected due to technical difficult ies so the total number included in this analysis is 35.

16 of the participants were female and 19 were male. Part icipants were screened in accordance with the target market identified by iRiver to be between 23 and 45 years old; recruited participants fell predo minately into the <28 (49%) or 29-33 (34%) age groups. All listened to music on a dedicated player or mobile phone and none had used the iRiver Spinn before.

Task-orientated trials, typical o f usability trials, can be an effective way to demonstrate the product to a participant in a controlled manner and the participants were encouraged to 'think aloud' during the study to communicate their thought process [START_REF] Du Mas | A Pract ical Guide to Usability Testing[END_REF]. Five tasks were chosen to introduce the participant sequentially to the device and no time constraint was imposed for the tasks. The tasks were:

Task 1: Turn the device on Task 2: Find and play a specific track Task 3: Adjust the volume of the track Task 4: Stop the track and navigate to the first screen Task 5: Turn the device off Next, each participant was asked to scroll through the main menu t itles and discuss what they expected within each menu. This user-led exp loration ensured each partic ipant had the same knowledge of the features of the device. After which a semistructured interview sought to gain feedback about both the physical design and the users' interaction experience of the product. The explicit nature of the tasks and user-led explorat ion is one of the recommendations to redu ce the evaluator effect on studies [START_REF] Hert Zu M | The Evaluator Effect: A Ch illing Fact About Usability Evaluation Methods[END_REF].

Finally, users were introduced to all the prototypes and asked to fill in a questio nnaire ranking the quality of feel, appearance and quality of interaction for each of the prototypes. This enabled the participants to directly compare prototypes and offer an opinion about their construction.

Participants were bought into a controlled environ ment and the entire user trial was recorded on video. A facilitator ran the study with an observer monitoring the study via the video link. The observer was able to ensure continuity across the studies; this was deemed more suitable than introducing them as a second evaluator due to their level of experience with the prototypes and user testing methodologies. The Facilit ator has conducted a number of similar studies before in a research and commercial context and is therefore able to reflect on techniques with colleagues of similar exp erience. Thus although the evaluator effect cannot be eliminated, it has been considered for this study [START_REF] Hert Zu M | The Evaluator Effect: A Ch illing Fact About Usability Evaluation Methods[END_REF].

Results of the user trial

The analysis was performed by the facilitator. Discourse analysis provided a framework to analyse the video footage of the tasks, menu exp lorat ion and semistructured interview. The strength of this approach is that it gives the ability to structure the conversational feedback typical of this type of study in a rigorous manner. The video footage was reviewed with event logging software and co mments were assigned 'codes' based on the type of co mment. 50 co mment groups were recorded in total. In order to co mpare the prototypes comments made by just one participant were removed. These comments were then rev iewed and co llated to form high-level design recommendations typical of a report fro m user trials [START_REF] Molich | Making Usability Reco mmendations Useful and Usable[END_REF]. Further reco mmendations could be drawn fro m the data produced by the studies that would be used in a co mmercial context. For the purpose of this study only the comments that have emerged through the formal discourse analysis are included. It is important to note that the recommendations themselves are not important to this study and have therefore been simp lified for this paper; it is the nu mber o f reco mmendations identified for each prototype in relation to the final device that is of importance in this context. The ten key co mments that the design recommendations address are:

1. Help required fro m the facilitator 2. Difficult ies in finding the required interaction 3. Tried other interactions 4. Pressed back to stop track playing 5. Tried turning dial to get to pause icon 6. Observation that it looks like a touch screen device 7. Like the 'Sp inn' interaction 8. Long-winded interface 9. No unique selling point 10. Vertical menu navigation not obvious Fig. 11. The ten key comments addressed by the design recommendations Figure 12 shows the results of the ranking exercise where each of the participants were introduced to all the prototypes and asked to give a rat ing where 6 is positive and 1 is negative. The participants were asked to rate three elements of the prototypes; the 'quality of feel' and 'appearance' which aimed to pro mpt the participant to consider the passive physicality ele ments and the 'quality of interaction' roughly equates to active physicality. Although these terms cannot be directly described as active and passive physicality, it goes some way to enable a comparison to the assessment of physicality shown in Figure 10. The data fro m the prototype the participant used for the study was not included to eliminate any bias fro m familiarity with the prototype. Figure 12 shows participants consider the foam prototype to have a low ranking but roughly equal for both elements wh ich supports our assessment of the prototype to be low in both active and passive physicality. Likewise the appearance and Arduino are ranked in a similar way to our assessment. The IE4 gives interesting results with it being considered a higher quality of interaction than the Arduino and a more marked difference between active and passive physicality than anticipated. It could be that the visual aspects of physicality are undervalued in the current definition of passive physicality or that these questions are not adequate at obtaining participants views of a ctive and passive physicality, this is beyond the scope of this paper but could be an interesting topic for further research. Th is exercise enabled participants to reflect on 1: 2: ie4 3:

4: arduino 5: final the prototypes themselves during the ranking exercise and the comments made were also captured, these will be brought into the discussion.

Limitations of the study

This study is recognized to have limitations that could be addressed in future work. The study has been designed, conducted and analyzed by one of the authors; therefore presumptions concerning active and passive physicality will inevitably influence the outcomes. Future work would seek to determine if the notion of active and passive physicality are applicable beyond this study. This is planned in a number of ways; firstly by re -evaluating studies conducted prior to the active and passive physicality notion, secondly by seeking discussion with those involved with interactive prototy ping fro m an academic and co mmercial context, and finally by evaluating future stu dies conducted by colleagues.

Discussion

In Figures 11 and12 the IE4 prototype appears to give feedback that is closest to the final iRiver device. These will be discussed along with other, more subtle, differences across the prototypes bringing in co mments fro m the ranking exercise. Observations fall into two categories; recommendations about the d esign and obstructions caused by the prototype. Recommendations positively help identify how the design can be improved wh ilst obstructions are caused by features of the prototype that hinder participants in giving mean ingful feedback.

Recommendati ons about the design

Physicality of the di al. The IE4 prototype was the only prototype that highlighted participants trying to turn the dial to get to the pause function. The physicality of the dial itself could be the cause of this, for the IE4 each rotation has a distinct 'click' wh ich causes a reaction in the interface. However the Arduino prototype did not produce this feedback and its dial had a similar physicality to the final device. This suggests that there must be something else about the prototype that causes the participant to miss feedback for this design recommendation. Several users made co mments about the wires of the Arduino prototype being "very distracting" and looking "messier" than the other prototypes, this 'messier' appearance could possibly be the cause of this.

Information architecture.

The feedback that the interface was longwinded was a common comment fro m participants of the trial with the final device. The IE4 and Appearance model were both good at drawing the same feedback. The Foam prototype was not able to elucidate this, possibly because the participant was not directly manipulat ing the prototype and therefore not creating the direct mental link between the physical and digital 'I did not like the fact that I couldn't control the device (interface) fro m the model'. Meanwhile the Arduino prototype produced few co mments about this possibly because the novelty of the prototype itself suppressed the participant's potential frustration with the navigation of the interface "this th ing (dial) works alright. I quite like the ability to click". The IE4 seems to g ive a very d irect feel between the interface and interaction , mimicking the final device well. The Appearance model forced the participant to have to continually press the scroll button to navigate the interface, high lighting the sheer number of button presses required to navigate the interface "Very tedious going through all the songs like this".

Obstructi ons caused by the prototypes

Modeling physical interfaces on a touch screen.

The Appearance model used a touch screen for the interactive element of the prototype. This prototype gave participants the least difficult ies in finding the interactions. Due to the need to represent all the buttons on a touch -screen this prototype clearly indicated where interactions were, even when they were on the side of the device. This made the interactions more obvious for those using this prototype than would otherwise have been. Parado xically, the very usability of the touchscreen prototype devalues it given the issues users had with the real device.

Obstacles to the partici pants understanding the prototype.

Figure 11 shows the Foam and Arduino prototypes forced participants to ask for the most help fro m the facilitator. The Foam model requires the part icipant to fully engage with the 'speak aloud protocol' because the buttons provide no active fee dback. The participant therefore has to wait for the facilitator to operate the interface. In contrast, the Arduino prototype allows the participant to operate it independently, but it may be that the appearance of the wires that seem to the biggest barrier to ac-ceptance. It may also be that techniques which require the participant to understand the way in which the prototype works are not suited for this type o f early stage trial.

Overview of the four prototypes

The IE4 prototype.

The real-time nature and simp licity of this prototype seem to be the important factors in making this prototype the most effect ive of the prototypes. Participants were able to operate and receive immed iate feedback fro m the interface without an overly complicated looking prototype or altering the scale and form of the model. "I felt very litt le difference in terms of the final version and white model (IE4) for the quality of interaction -white model (IE4) had a few blips but nothing that is stopping me using the device successfully." "The addition of working buttons on the prototypes increases the quality of the feel, as the ways in wh ich interaction occurs can be mo re readily envisioned."

The Foam prototype.

This prototype used the 'speak out loud' protocol for participants to engage with the interface. Results show that this prototype was less effective at enabling partic ipants to build a mental model of the device resulting in reduced effect iveness of the comments received. "The co lour, weight, size and cable connections play a big part of my initial interaction with a product, for this reason the blue foam co mpared to the final unit was clearly a visual aid as opposed to actual real product comparison."

The Arduino prototype.

Participants required more assistance using this prototype. This was a surprise from the most interactive of the prototypes. Participants seemed to be affected by the wires and appearance of this prototype. "The model with blue foam & wires looks messier than the blue foam model but it looks a little bit more functional than the model with blue foam alone."

The Appearance prototype .

This prototype used a touch screen to convey the interactions of the prototype. Pa rticipants did not identify as many usability errors and had the weakest performance in relation to the final device. This outcome supports Gill's study in wh ich it was proposed that interactions are easier for a participant to identify on a screen [START_REF] Gill | Rapid Develop ment of Tangib le Interactive A ppliances: Achieving the Fidelity / Time balance[END_REF]. "Although the silver model (appearance model) looked more like the final version, it d id not like the fact that I couldn't control the device fro m the model and I d idn't think having the model alone, without much interaction, was very worthwh ile."

Conclusion and Application

The four prototypes trialled in this study explored different aspects of active and passive physicality. The results show that both active and passive physicality are i mportant considerations for early stage user feedback; but it is an even balance of these that produces the most effective prototypes, as seen in the IE4 and Foam prototypes. Resources should not be used exclusively to ensure the prototype functions well in an electronics and interaction sense (active physicality) if it severely impacts the ways the prototype looks or can be held by the user (passive physicality). Likewise, resources spent creating a prototype that looks very close to a final device are not effe ctive if interactions are not well supported.

The IE4 and Foam prototype provided the most accurate data compared to the user experience of the real device. Both the IE4 (£ 760) and Foam prototype (£60) were of balanced physicality. The Arduino (£1,100) was very strong on active physicality to the detriment of passive physicality wh ilst the Appearance model (£1,160) was very high on passive physicality but low on active physicality. This suggests that it is those prototypes that are well balanced that are the most effective in this study. Since they are also cheaper they represent strong value for money. 2 The prototype has long been accepted as a valuable approach to creating valuable and insightful design outputs . However, for interactive devices that have both a phy sical and digital form, visual fidelity alone is clearly not enough to fully conceive the complete prototype and ensure it will accurately fu lfil its purpose. Whilst visual and dimensional fidelity is very much the staple of prototyping, physical fidelity clearly has a role in creating a well-targeted prototype. This study indicates that for interactive prototyping, 'physicality' needs to be an even combination of active and passive physicality.

7

Future Work

Future work needs to be conducted to determine if active and passive physicality can be usefully used in assessing prototypes beyond those used in this study. The outcome of this study indicates that a balanced prototype is the most effective . The prototypes used in previous studies [START_REF] Gill | Rapid Develop ment of Tangib le Interactive A ppliances: Achieving the Fidelity / Time balance[END_REF] [START_REF] Hare | Physical Fidelity: Explo ring the Importance of Physicality on Physical-Digital Conceptual Prototyping[END_REF] should now be assessed in terms of physicality to determine for examp le if notions of active and passive physicality aid in determining why the data for the 'flat-face' prototype differed considerably fro m the final device. In addit ion prototypes used in studies by other authors could be categorized to see how they relate to our prototypes.

Fig. 5 .

 5 Fig. 5. Prototype 1: Foam prototype

Fig. 8 .

 8 Fig. 8. Prototype 4: Arduino prototype

Fig. 10 .

 10 Fig. 10. The resulting physicality of each of the prototypes

Fig. 12 .

 12 Fig. 12. Data from the ranking exercise; comparing the prototypes

The IE4 is a wireless device which converts buttons presses into keyboard presses