G Michael Poor

Guy Zimmerman

Dale S Klopfer
email: klopfer@bgsu.edu

Samuel D Jaffee
email: jaffees@bgsu.edu

Laura Marie Leventhal
email: leventha@bgsu.edu

Julie Barnes
email: jbarnes@bgsu.edu

Mobility Matters: identifying cognitive demands that are sensitive to orientation

Keywords: Mobility, Mental Rotation, Presentation Orientation, Spatial Ability

Prior studies have shown benefits of interactions on mobile devices. Device mobility itself changes the nature of the user experience; interactions on mobile devices may present better support for cognition. To better understand cognitive demands related to mobility, the current study investigated presentations on a mobile device for a three-dimensional construction task. The task imposed considerable cognitive load, particularly in demands for mental rotation; individual differences in spatial ability are known to interact with these demands. This study specifically investigated mobile device orientations and participants' spatial ability. Subjects with low spatial ability were able to complete the task more effectively when shown the presentation in a favorable orientation. Individuals who saw the presentation in an unfavorable orientation and those of low spatial ability, were differentially disadvantaged. We conclude that mobility can reduce cognitive load by limiting demands for spatial processing relating to reorientation.

Introduction

Recently there has been an enormous expansion in the sale and use of mobile devices as information appliances. It is reasonable to ask why, or more specifically, what is it about mobile devices that make them attractive, as compared to fixed stationary devices. There are a number of obvious answers: size, convenience or price. However, if users did not find the mobile device to be at least as useful as a fixed counterpart, would these devices be so successful? It seems likely that that users experience mental workload advantages as an outcome of the very mobility of the mobile device for some tasks. For example, extracting and interpreting instructions for construction tasks 1 , tasks that impose considerable mental workload in the form of mental rotation and spatial processing on users, would fit this description.

In this paper, we first review background literature, including a discussion of the likely cognitive load issues at play in construction tasks on mobile devices, specifically mental alignment of the presentation to the built object, achieved via mental rotation. We present a study that examined the role of physical device orientation on performance on a construction task. Additionally, in the study, we explored the role of an individual difference variable, spatial ability, on performance of the construction task. Finally, we present the results and we discuss the ramifications of our findings for designers.

While the constant changes in technology make the definition of mobile device or mobile interaction moving targets, for the purposes of this paper the terms mobile device or mobile interaction will imply a handheld computing device possessing a display screen and input mechanism. This definition includes cell phones, smartphones, tablets, handheld GPS (global position) systems and PDAs (personal digital assistants), but excludes traditional desktop computers with fixed displays. The key defining feature that we focus on in this paper is the ability of the user to easily reposition the display device in any desired orientation. For clarity, we will use the term mobile device.

2

Background Literature

Mobility Matters

It is widely believed that different interactions engage different user capabilities and draw on different elements of human cognition [cf. 12]. More specifically, researchers have noted numerous HCI issues for mobile devices [cf. [START_REF] Abowd | Charting Past, Present, and Future Research in Ubiquitous Computing[END_REF][START_REF] Bouri | Augmenting Spatial Skills with Mobile Devices[END_REF][START_REF] Dunlop | The challenge of mobile devices for human computer interaction[END_REF][START_REF] Mohamedally | Recent research in mobile computing: a review and taxonomy of HCI issues[END_REF]. Of interest here, [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF][START_REF] Zimmerman | A comparison of the usability and effectiveness of web-based delivery of instructions for inherently-3D construction tasks on handheld and desktop computers[END_REF] found that people used differential strategies to varying degrees of success when performing a three-dimensional construction task using instructions presented on mobile and non-mobile devices. For these tasks, the instructional presentation included interactive 3D models. Traditionally, the instructions for completing construction tasks are presented on paper, with written directions often annotating visual representations of the assembly process. Such paper based instruction presentations are notoriously difficult to use. [START_REF] Zimmerman | Delivering instructions for inherently-3D construction tasks: an evaluation of authoring environments for multimedia presentations[END_REF] has suggested that difficulties arise in part from task demands for mental rotation. Interactive presentations offer relief from some of the limitations of the traditional paper format. In particular, interactive presentations allow the three-dimensional displayed object to be viewed from any vantage pointgiving the builder a better sense of the spatial relationships of the parts of the assembly. A number of factors potentially impact performance on construction tasks, including the nature of the presentation and the spatial ability of the participants [START_REF] Poor | The effects of varying levels of reality based interaction on a subjects' ability to perform a 3D construction task[END_REF].

More importantly for the current study, instructions on a mobile device allow users to physically take the instructions 'to the object': physically orienting the instructions by holding the device proximate to a built object. Further, the richness of construction tasks would seem to make them ideally suited to highlight mental workload differences between mobile and non-mobile devices. [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF][START_REF] Zimmerman | A comparison of the usability and effectiveness of web-based delivery of instructions for inherently-3D construction tasks on handheld and desktop computers[END_REF] compared performance on a construction task between a mobile device presentation and a fixed upright display presentation. They found that the mobile device users were more efficient in building the target object than the fixed presentation users. [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] also found that at least 25% of the persons with the mobile device employed a strategy of moving and aligning the mobile device to the object being built during at least one building step and all but one participant removed the mobile device from its starting position during the building process. Interestingly, in [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] a number of participants with a stationary display brought the object being built to the display. In other words, in both conditions, participants aligned the physical device with the object being built. When the person could bring the presentation to the object instead of vice versa, performance was markedly improved. [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] concluded that the participants using the non-mobile presentation found the process of aligning the object to the screen was awkward, forcing them to mentally rotate in order to realign the images in the instructions to the constructed object. The participants could have interactively realigned the 3D presentation, in either device condition, at any point and it is possible that the subjects in [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] did this; it is notable that the mobile presentation users had better performance regardless.

Does Orientation Matter?

When a person is following computerized instructions that include visual presentations to construct an object, they have several choices as to how to align the spatial relations in the visual representation to those of the target object. They can physically move the presentation to the target via the mobility of the device, physically align the target to the presentation, manipulate the presentation of the digitally displayed 3D object, and/or perform any or all these operations mentally, without manipulating the object or the presentation. In other words, in a construction task, when the visuals in the presentation and the actual built object are misaligned, the user will mentally, physically, or interactively perform transformations to make the alignment. [START_REF] Zimmerman | Delivering instructions for inherently-3D construction tasks: an evaluation of authoring environments for multimedia presentations[END_REF]'s results suggest that users are most successful when they choose to physically realign the device to the target and that they may be surprisingly unwilling to realign interactively.

As we consider the fact that in [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF], mobile device subjects were able to move the device to realign the images in the presentation to the target while the fixed desktop subjects appeared to more often do this mentally, the next obvious question should be, does it matter? If desktop subjects are doing more mental rotations of the presentation, is there a cost? [START_REF] Shepard | The mental image[END_REF] claimed that internal (cognitive) representations share a second-order isomorphism to the world they represent. One outcome of this conjecture is that the greater the angular disparity between the starting orientation of an object and its rotated position, the more effort required for rotation of the object both in the real world and in their internal representations [START_REF] Shepard | Upward direction, mental rotation, and the discrimination of left and right turns in maps[END_REF][START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF]'s finding of perfor-mance advantages for mobile device users suggests that the mobility of the device may reduce user cognitive load by reducing need for mental rotations.

Some studies indicate that cognitive load increases as a person does more mental rotations [e.g. [START_REF] Just | Cognitive coordinate systems: Accounts of mental rotation and individual differences in spatial ability[END_REF][START_REF] Klopfer | Working Memory: Just & Carpenter[END_REF]. That some participants in [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] aligned the object that they were creating with the image on the fixed display whereas others aligned the mobile device with their built object highlights an obvious but critical difference between fixed vs. mobile display devices: mobile devices allow the user to change the orientation of the display, which can change the frame of reference used to specify spatial features of an object, such as identifying its top or its left or right side. In the fixed display condition in [START_REF] Zimmerman | Delivering instructions for inherently-3D construction tasks: an evaluation of authoring environments for multimedia presentations[END_REF], participants' options for rotating the presentation to align it with an external object were: mental rotation, rotation of the real object relative to the fixed presentation and/or rotation of the interactive 3D presentation. In the mobile device condition of [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF], subjects had a fourth optionthey could rotate the device containing the presentation.

When an observer encounters an object in the world, two frames of reference --and the spatial relations they define --are important to consider. First, there is an egocentric reference frame that defines spatial relations from the observer's viewpoint (e.g., up/down, left/right). The egocentric up/down axis is typically defined by gravity, with left/right defined by what's to the left and right of the viewer's midline, respectively. Because of the invariance of gravity, the up/down axis is a primary reference frame for defining the tops and bottoms of objects and whether one object is above (or below) another [cf. [START_REF] Shiffrar | Comparison of cube rotations around axes inclined relative to the environment or to the cube[END_REF].

There is also a reference frame intrinsic to the object itself whereby spatial relations among parts of the object are specified. Object-centered reference frames can be defined by a variety of object characteristics, such as an object's focal point [START_REF] Braine | On the Relation Between the Natural Logic of Reasoning and Standard Logic[END_REF], an axis of symmetry or elongation, or surface markings [START_REF] Palmer | Vision science: Photons to phenomenology[END_REF]. The object in Figure 1(a) has an intrinsic axis of elongation, defined by the dotted line; the triangle in Figure 1(b) has an intrinsic axis of symmetry and a focal point at its upper vertex. With both objects, the intrinsic axes are aligned with the egocentric up/down axis. If these were animate objects, people would likely construe the upper portion of each object to be its head; if they were to move they would move upwards. The triangle in Figure 1(c) is probably seen as pointing up, illustrating the primacy of the egocentric up/down axis. With its three axes of symmetry, the triangle in Figure 1(c) actually points in three directions, but the tendency to see it point up is due, in part, to the viewer using the up/down axis to assign spatial relations.

When an observer encounters an object displayed on a screen, a third frame of reference comes into play: the reference frame defined by the edges of the display. Figure 2 shows the triangle from Figure 1(c) surrounded by a rectangular frame, much as how the triangle would be seen on a desktop display. Note how the two reference framesthe egocentric up/down axis and the vertical axis of the displayare aligned, and the triangle is seen as pointing up. Due to the alignment of the viewer's and the display's reference frames, a desktop display's reference frame is redundant, providing the same spatial relations as the viewer's up/down axis. With a mobile device, the reference frame defined by the display need not be aligned with the viewer's up/down axis. Figure 3 shows the triangle from Figure 2 within a rotated rectangular frame. Here, there is a strong tendency to see the triangle pointing down to the left, although it still is possible to see it pointing up. That is, by changing the display's frame of reference, the "head" of the triangle shifts from being the upper vertex to the one in the lower left. Moreover, within the context of the rotated frame, the triangle is likely seen as heading strictly to the left instead of down and to the left.

Fig. 3. Triangle inside of rotated display

As noted earlier, the mobility of mobile devices allows one to alter the display's frame of reference quite easily. Indeed, the rotated rectangular frame in Figure 3 represents just one of an infinite number of display-defined frames of reference available to the mobile device user. The interactive graphics that allow for the shape to be rotated within the display gives the user freedom to define the alignment of the presentation using whatever reference frame he or she chooses.

In the construction task in [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] there is a fourth frame of reference to consider: the one that defines the top, bottom, left, or right of the target object being constructed. In principle, there are more opportunities for the four reference frames (i.e., viewer, display, displayed object, constructed object) to be misaligned when using a mobile device than when using a fixed desktop display (i.e., viewer = display, displayed object, constructed object). Much research in cognitive psychology indicates that there is a cost in perceiving objects when viewer and object frames of reference are misaligned [cf. [START_REF] Jolicoeur | Orientation congruency effects on the identification of disoriented shapes[END_REF][START_REF] Shiffrar | Comparison of cube rotations around axes inclined relative to the environment or to the cube[END_REF]. Thus, misalignment of two frames of reference, viewer and object, can have information processing costs. To our knowledge, no research has been done that examines how users cope with the possibility of there being multiple opportunities for misalignment.

Evidence of the Importance of Presentation Orientation

Even with the possibility of physically reorienting a presentation, determining the best interactive realignment may in and of itself impose significant cognitive load and involve mental rotations in planning, especially for low spatial ability users. Given that many contemporary mobile devices only automatically realign in cardinal directions, should it turn out that physical orientation and spatial ability do interact on a construction task, persons of low spatial ability who cannot physically reorient to the best orientation will be disadvantaged unless the cognitive load for interactive realignment can be reduced. Three older studies point to the importance of orientation in a presentation of interactive visual information. [START_REF] Seager | Comparing Physical, Automatic and Manual Map Rotation for Pedestrian Navigation[END_REF] found that, for map-based navigation assistance, physical rotation is the most effective form of track-up alignment on handheld mobile devices This was due to the users' difficulty to recognize a map when automatically rotated, especially when the users were not looking at the map during the time of rotation.

In addition, [START_REF] Tang | Comparative effectiveness of augmented reality in object assembly[END_REF] described a comparative study of the effectiveness of four different presentations of instructions for an assembly task: printed manual, monitordisplay, see-through head-mounted display, and spatially registered augmented reality (AR). Measurements were task performance (time and accuracy) and perceived mental workload. The task consisted of 56 procedural steps building an object with Duplo blocks. Participants in the spatially registered AR treatment made significantly fewer assembly errors. The authors concluded that the improvement in the AR condition was due to reduced demand for attention switching. Because the spatially registered AR appears directly on the object, it was also thought that the participants did less mental transformations between the instructions and the object. [START_REF] Eisenberg | Helping users think in three dimensions: steps toward incorporating spatial cognition in user modeling[END_REF] reported on a design tool to build three-dimensional, interactive and movable polyhedrons. In evaluating this tool, they found that users had a preferred orientation for the designed polyhedrons. When the figures were moved from the preferred orientation, subjects found them to be more difficult to sketch (reproduce by hand). Some participants reported elements of the preferred orientation include: 1) preference for vertical as opposed to horizontal edges (preference for either type of edges as compared to diagonal edges), 2) bilateral symmetry, and 3) stability as indicated by the polyhedron resting on a face as opposed to resting on a vertex.

The Role of Spatial Ability

Performing mental transformations, such as those described by [START_REF] Just | Cognitive coordinate systems: Accounts of mental rotation and individual differences in spatial ability[END_REF] can impose a workload on working memory. In particular, the mental rotation processes can be time-consuming and error prone, particularly as the complexity of the object being rotated increases and its familiarity decreases [START_REF] Bethell-Fox | Mental rotation: Effects of Stimulus complexity and familiarity[END_REF][START_REF] Folk | Effects of stimulus complexity on mental rotation rate of polygons[END_REF]. Just how much effort the mental rotation processes require also depends upon an individual's spatial ability, i.e. the ability to generate, retain, and transform well-structured mental images [START_REF] Lohman | Spatial abilities as traits, processes, and knowledge[END_REF][START_REF] Lohman | Spatial ability and G[END_REF]. Individual differences in spatial ability are related to individual differences in working memory function [START_REF] Lohman | Spatial ability and G[END_REF][START_REF] Miyake | How are visuospatial working memory, executive functioning, and spatial abilities related? A latentvariable analysis[END_REF], with transformations such as mental rotation taking longer for users with lower spatial ability, as measured by paper-and-pencil standardized spatial ability tests [START_REF] Ekstrom | Kit of Factor Referenced Cognitive Tests[END_REF]. Mobile devices potentially provide a means for users to align a displayed object with their own egocentric up/down, limiting the need to engage in mental rotation in order to achieve alignment. The savings would be greater for those with lower spatial ability and would potentially expand the usefulness of the device to a larger population.

Summary: Background Literature

Prior research has suggested four intersecting themes: 1) Performance on a construction task is better with a mobile device than on a fixed display device 2) Construction tasks engage mental workload, much of which is involved in mental rotation to align disparate frames of reference. The cost of mentally aligning an egocentric and presented object-centric frame of reference is known to be high; the cost of realigning those frames of reference plus others from a display and a built object are not known 3) Mental rotation requires significant mental workload and 4) People differ in their abilities to perform mental rotation; those of lower spatial ability, as measured on standardized tests, find spatial tasks like mental rotation, more difficult than those of higher spatial ability. Taking these themes together, we suggest that performance on a construction task is better with a mobile device as compared to a fixed display device because the mobile device participants are able to lessen some of the mental work of aligning the presentation to the object to be built. In [START_REF] Mohamedally | Recent research in mobile computing: a review and taxonomy of HCI issues[END_REF] mobile device participants accomplished the needed rotations by a combination of mental and physical rotations rather than mental rotation alone. We hypothesize that mental rotation, interactive rotation of the presentation, or rotation of the artifact is more difficult than rotating the device itself. We suggest that when rotating physically, with the immedi-ate visual feedback as the virtual and physical object align, the participant does less mental rotation, thus reducing their mental workload.

Recognizing that people differ in their ability to do mental rotation, those of lower spatial ability, should be differentially more impaired with a stationary device - [START_REF] Mohamedally | Recent research in mobile computing: a review and taxonomy of HCI issues[END_REF] did not measure the spatial ability of their participants, so we cannot be sure of this conjecture from their results relating to spatial ability.

In the current work, we start with the assumption that part of the power of a mobile device comes from the reduction in necessary work of mental rotations. We speculate that this advantage may extend further for those who are more challenged by deficits in their ability to perform mental rotations. In our study, we seek to demonstrate that having the visual presentation for a construction task aligned in a particular way, as one would be able to do with a mobile device, would lead to superior performance, than having the presentation in other orientations.

Study: Impact of Orientation of Presentation and Spatial Ability on Construction Task Performance

For this study our hypothesis is: Orientation of a mobile device, in combination with participant spatial ability will affect performance on a construction task. We specifically hypothesize that at least one orientation will lead to better performance on the construction task by leading to fewer differences in frame of reference between the built object and the presentation by reducing mental rotations (and lowering cognitive/working memory load). However, because the built object itself is in a number of orientations during the presentation, we do not predict which orientation(s) would be favorable and which would impede subjects' performance on the task. Following prior work on the interaction between spatial ability and mental rotation, we also predicted that persons of low spatial ability would be differentially hampered in the less favorable orientations.

Experimental Design

We have two independent variables: Mobile Device Orientation and Spatial Ability, and two dependent performance variables, described in Section 4, relevant to the task. [START_REF] Mohamedally | Recent research in mobile computing: a review and taxonomy of HCI issues[END_REF] found that time on the task was non-informative; it was not considered as a dependent variable.

3.2

Mobile Device Orientation.

In the study, the mobile device was physically anchored in four orientations (denoted: left, right, top, bottom) as shown in Figure 4a. Figure 4b shows the experimental setup with the device in the right orientation. The presentation of the instructions for the construction task was symmetric relative to both vertical and horizontal orientations and we collapsed the orientations into two categories: UpDown and LeftRight. While fixing the mobile device may seem counter-intuitive (removing the "mobile" aspect of the device), we have done so in order to allow for greater experimental control in order to study the effects of the frame of reference imposed by the display of the mobile device.

The participants sat upright at the table facing the instructions and were to the extent possible, in a fixed egocentric orientation; participants' position was set so that they did not reorient the presentation by reorienting their own viewpoint.

Spatial Ability

We measured spatial ability using the Card Rotation task, a measure of twodimensional mental rotation [START_REF] Ekstrom | Kit of Factor Referenced Cognitive Tests[END_REF], an individual ability that should be at play, at least in part, in our construction task. Because this is not a power test (i.e., the task does not get harder at the end) the scores were calculated by subtracting the total number of wrong responses from the total number of correct responses. Our median subject score was 69. The minimum and maximum scores were -58 and 154 respectively. Using median split, participants were grouped into two categories: high and low spatial ability.

3.4

Participants

Thirty-two participants, drawn from undergraduate computer science classes, completed the task to their satisfaction with 16 persons in the UpDown condition and 16 persons in the LeftRight Condition. Two participants from each spatial ability category were dropped; they were the participants closest to the median spatial score of 69, leaving a pool of 28 participants with 14 participants in each orientation category. In terms of spatial ability, this change to the pool left 15 participants with high spatial ability and 13 with low spatial ability. Nine participants were assigned to the High spatial ability/UpDown condition, five participants to Low spatial ability/UpDown, 6 participants to High spatial ability/LeftRight and eight to Low spatial ability/LeftRight. A chi-square analysis of this frequency distribution was not significant, showing that the assignment to condition was independent from spatial ability.

3.5

Materials and Task

In previous studies we explored the effectiveness of interactive 3D graphics as a part of a system to deliver instructions for a construction task: origami paper folding [cf. [START_REF] Barnes | Look and touch: the impact of touchscreens on the delivery of instructions for inherently 3D construction tasks using web-delivered virtual reality[END_REF][START_REF] Zimmerman | Delivering instructions for inherently-3D construction tasks: an evaluation of authoring environments for multimedia presentations[END_REF]. Paper folding does possess many representative characteristics of construction tasks: the task is non-trivial, it requires multiple manipulation steps, and it results in a 3D artifact. Researchers in multimedia learning make a distinction between single and dual presentations of instructional information [START_REF] Mayer | Multimedia Learning[END_REF][START_REF] Paivio | Mental representations: A dual coding approach[END_REF]. In single presentations, instructions are typically presented in text alone, whereas in dual presentations instructions are usually presented in text with accompanying still images or other representations [START_REF] Paivio | Mental representations: A dual coding approach[END_REF]. A number of studies indicate that dual presentations lead to better performance [e.g. [START_REF] Mayer | Multimedia Learning[END_REF][START_REF] Teasley | Effective illustrations in interactive media: what works? In: Human-Computer Interaction[END_REF]; this advantage has been shown with the present task [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF]. Our task, identical to the task used in [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF], was to fold an origami whale in 25 paper folds (and unfolds), with the instructions for making the folds presented in a series of 12 steps. Approximately ½ of the steps involve 2D folds, the remainder were 3D folds, including steps to form the mouth, fins and tail. A completed whale is shown in Figure 5. The IPAQ and the interaction user interface were much less familiar to the participants than a smartphone or handheld GPS system; it was our thought that participants were not able to engage familiar tasks, outside of the specific experimental task, during the experiment. The 3D interactive presentation was implemented using VRML 2.0 (Virtual Reality Modeling Language). The VRML model was rendered within Pocket Internet Explorer 5.5 using the Pocket PC Cortona VRML client plug-in. The display screen was 3.8" (diagonal) with resolution 240x320 with 16-bit color. There were two visual components to the display: the virtual sheet of origami paper (VOP/model) and the user animation/step interface. A simulated sky/horizon was also implemented to provide a spatial frame of reference for the origami paper. The user moved through the steps by clicking the forward/backward buttons on the interface. During each step the user could start/stop an animation of the desired operation (e.g., create a fold) by clicking the play/pause button. For each step that required a fold operation, the animation began by highlighting the desired fold line on the VOP; the actual fold operation was then performed on the VOP. The user also had the ability to rotate the VOP/model in any direction at any time. Technical VRML implementations details can be found in [START_REF] Mohamedally | Recent research in mobile computing: a review and taxonomy of HCI issues[END_REF]. Figure 6(a) shows the 3D interactive presentation. Subjects had access to written instructions with figures printed on paper; these were anchored in front of the subject using a "flipchart" style of presentation (see Figure 6

Procedure

The subjects completed the spatial ability test and a training task to familiarize them with the interactive controls. Then they completed the whale-folding task of twelve folding steps, used in [START_REF] Zimmerman | Delivering instructions for inherently-3D construction tasks: an evaluation of authoring environments for multimedia presentations[END_REF][START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF][START_REF] Zimmerman | A comparison of the usability and effectiveness of web-based delivery of instructions for inherently-3D construction tasks on handheld and desktop computers[END_REF].

3.7

Scoring of the Folded Whales.

Each participant's whale was scored by evaluating every fold on the origami paper. Each fold was scored as Correct, Error and/or Recrease. If a subject performed a fold incorrectly and then folded correctly, these were scored as different folds (one correct and one error). In addition, a correct fold might be recreased. The range of Correct Folds was 0 -25; of Error Folds and Recrease Folds was 0no maximum. Two people graded the constructed origami whales with an inter-rater reliability of 0.99.

3.8

Summary: Relating the Study to the Hypothesis [START_REF] Zimmerman | How Do I Line Up?: Reducing Mental Transformations to Improve Performance[END_REF] posited that construction tasks in general and the one used in this study specifically impose significant mental workload as the participants must align four frames of references (themselves, the presented instructions, the display and the built object) using a combination of mental and physical rotations. In this study, physical rotation of the display and rotation of the egocentric (participant's) frame of reference were limited or fixed by the experimental setup. Realignments were possible through the interactive rotation of the presentation, physical movement of built object and mental rotation of any of the component elements. All participants were free to rotate the interactive presentation or to move the built object. Only demands for realignment by mental rotation differed by orientation. We expected to find that persons of high spatial ability would outperform those of low spatial ability across the board, simply by the fact that the cost of mental rotation is higher for individuals of low spatial ability.

Our alternative orientations simulate the various positions that a mobile device could be in. If one orientation leads to better performance, especially for persons of low spatial ability, then we would have shown that physical alignment of the display to the other experimental components does reduce mental workload on our task.

Results

We considered the impact of the independent variables, Orientation (UpDown vs. LeftRight In order to understand these findings in detail, we examined the two components of Adjusted Number of Correct Folds (viz., the Number of Correct Folds and the Number of Error Folds) separately as a function of Orientation and Spatial Ability. High spatial ability participants made more correct folds than those with lower spatial ability (F(1,24) = 6.349, p < 0.019), and participants in the LeftRight orientation made more correct folds than those in the UpDown orientation (F(1,24) = 4.678, p < .041). The Orientation X Spatial Ability interaction was not significant for Number of Correct Folds. The Number of Errors, on the other hand, was significantly greater in the UpDown than in the LeftRight orientation (F (1, 24) = 5.410, p < .029). There was also a main effect of Spatial Ability, with Low Spatial Ability participants making more errors than their High Spatial Ability counterparts (F(1,24) = 4.566, p < 0.043). The Orientation X Spatial Ability interaction was also significant for Number of Error Folds(F(1,24) = 4.424, p < 0.046), showing that the performance of participants with lower spatial ability was most affected by the unfavorable UpDown orientation. The means and standard deviations are listed in Table 2

Discussion: Study Results

Our results show significant disadvantages for participants who were in the UpDown orientation or low on a measure of two-dimensional rotational spatial ability. The disadvantages were exacerbated for persons who were both in the UpDown condition and had low spatial ability for the dependent measure, Number of Error Folds. The fact that persons of high spatial ability performed better on the construction task is not surprisingwe anticipated that the construction task imposed a higher cognitive cost for persons of low spatial ability. The fact that the two orientations led to differences in performance suggests that the position of a device imposes differential demands in mental work. With a real mobile device, individuals can reduce the workload by moving the device. That the LeftRight orientation was related to improved performance suggests that the critical elements of the presentation best lined up with the object being built in this orientation. Future studies could explore the specific elements of the presentation that was influenced favorably by the LeftRight orientation.

Vertical Orientation: Does it Matter?

In our study, we manipulated the orientation only in two spatial dimensions on a flat table surface. It is possible that the flat manipulation is not ideal and that the third dimension of vertical could be key as well. In a follow up pilot study we compared the Up subjects from the UpDown group to a group of participants who saw the presentation on the mobile device in a stationary vertical position. The setup is shown in Figure 7. All other aspects of the pilot study procedure were identical to our primary study, described in Sections 3 and 4. We chose to compare the Vertical orientation to the Up orientation from the tabletop conditions in our original study, because the Vertical presentation is also in the Up orientation but rotated 90 degrees vertically from the desk surface. We had eight subjects in the Vertical orientation (3 lows and 5 high spatial ability) and we compared this group to the original eight subjects from the Up (5 lows and 3 high spatial ability). We conducted a MANOVA with dependent variables Adjusted Number of Correct, Folds and Number of Correct Recrease Folds for the two independent varia-bles, Orientation (Vertical vs. Up) and Spatial Ability. The only significant effect, following the MANOVA was for Spatial Ability (Wilks' lambda= 0.561, F(2.0, 11.0) = 4.3, p < 0.042). Separate univariate ANOVAs showed that Spatial Ability had a significant effect on Adjusted Number of Correct Folds (F(1,12) = 9.377, p<.01) only. So the independent effect of 3D vertical orientation did not have a significant impact on performance nor did it interact significantly with spatial ability. The means for the two groups were low spatial ability participants= 13.567, high spatial ability participants = 23.3.

Discussion: Pilot Study Results

For the whale folding task, the subjects were not folding the physical whale vertically; the pilot study results suggest the vertical orientation of the mobile device did not align effectively with the physical whale. The Vertical mobile device alignment also did not yield different results from the Up tabletop position. We posit that Up orientation placed the interactive presentation in a mostly vertical orientation like the position of the display in the Vertical orientation of the pilot. The LeftRight orientation, superior to the UpDown orientation, made up in part by the Up orientation, likely positioned the salient elements of the interactive presentation predominantly in the position that participants favored during whale construction and that limited demands for mental realignment.

Conclusions

We found that user performance was significantly affected by the physical orientation of the mobile device, spatial ability and their interaction on a paper folding task. We make our first conclusionmobility does change the user experience at least for some tasks; enabling reorientation potentially reduces the need for mental rotations. While the study was not specifically designed to systematically control the orientations of the 3D interactive images shown, it appears that when making folds, performance was better when the model was aligned left and right. It is noteworthy that the subjects could have interactively changed the orientation of the 3D model at will. The fact that the orientation of the mobile device was a significant factor in performance suggests that the subjects did not, on at least some occasions, rotate the model to the more favorable orientation. This finding suggests that the frame of reference imposed by the display may have had greater power over the participants' mental representations of the task than the mental representation of the object itself. Prior studies have demonstrated that there is a cognitive cost for mental rotations effected to align disparate frames of reference. In order to select an interactive rotation would have required the subject to mentally rotate the model before interactively reorienting the presentation. Our results suggest that at least some of the time, subjects make tradeoffs between impaired performance and the mental effort required for mental rotations or for planning for interactive rotations. We conclude that mobility matters in part because mobility allows users to put presentations into favorable orientations and reduces the need for mental rotation.

Our results have implications for the design of presentations for mobile devices. We note that many contemporary mobile devices automatically alter the orientation of presentations on the screen, based on the physical position of the device. Should a person move a device to limit the mental work of rotating the presentation, only to have the device itself rotate the presentation, the automatic re-rotation could actually add to user workload.

Mobile devices and visual-spatial presentations of information are pervasive and likely to become more so, especially for tasks in which the mobile device can be moved to close proximity of the task [cf. 9]. Designers will be increasingly challenged to build user interfaces that do not inadvertently incorporate significant cognitive barriers to users in the form of memory load, especially for low spatial ability individuals. For individuals who are unable to physically reorient a device, our results suggest that they too may be potentially disadvantaged as they may be forced to rely initially on mental rotation to plan their interactive reorientation. Designers potentially may be able to expand the usefulness of their designs to broader spectra of the population by limiting the need for mental rotation via the mobile properties of the device.

Fig. 1 .Fig. 2 .

 12 Fig. 1. (a,b,c) Objects with vertical axes of symmetry

Fig. 4 .

 4 Fig. 4. a) Orientation conditions for the mobile device. b) Set up for right condition. Note: device is fastened to the table; paper instructions are anchored to the front of a monitor.

Fig. 5 .

 5 Fig. 5. The completed whale

  (b)).

Fig. 6 .

 6 Fig. 6. a) The 3D interactive presentation b) The experimental setup; note the built whale is misaligned with the presentation (inset).

  ) and spatial ability (High vs. Low) on two dependent variables: Adjusted Number of Correct Folds (defined as Number of Correct Folds minus Number of Error Folds) and Number of Correct Recrease Folds (multiple redundant correct folds). The subjects in this study did well on the task, as indicated by our Adjusted Correct Folds measure, with an overall mean of 16.3.As we had multiple dependent variables, we first conducted a multivariate analysis of variance (MANOVA)2 . The dependent variables (Adjusted Number of Correct Folds, and Number of Correct Recrease Folds) were included in the MANOVA. The main effects were significant; the interaction between Orientation and Spatial Ability was not. (Wilks' Lambda = 0.714, F(2.0,,23.0) = 4.603, p < 0.021 for Orientation; Wilks' Lambda = 0.672, F(2.0,23.0) = 5.607, p < 0.010 for Spatial Ability). A two-way ANOVA revealed significant main effects of Orientation and Spatial Ability on the Adjusted Number of Correct Folds [F(1,24) = 9.323, p < 0.005) and F(1,24) = 10.539, p < 0.003, respectively]. No univariate analyses of Number of Correct Recrease Folds were significant. The means of the dependent variables are shown in

Fig. 7 .

 7 Fig. 7. Mobile device in a fixed vertical orientation.

Table 1 .

 1 From the means for Adjusted Number of Correct Folds, persons with high spatial ability or the Left-Right orientation performed significantly better than persons with low spatial ability or UpDown orientations.

Table 1 .

 1 Mean Adjusted Number of Correct Folds and Correct Recrease Folds by Spatial Ability (High Spatial Ability vs. Low Spatial Ability) and Orientation (Updown vs. LeftRight) (standard deviations in parentheses)

  .

				Number of Correct Recrease
		Adjusted Correct Folds	Folds
		UpDown	LeftRight	UpDown	LeftRight
	High spatial abil-	18.00 (6.90)	21.50 (3.27)	4.33 (2.00)	2.83 (0.75)
	ity	n=9	n=6	n=9	n=6
	Low spatial abil-	7.00 (4.30)	18.00 (4.30)	3.60 (3.36)	4.88 (2.96)
	ity	n=5	n=8	n=5	n=8
	Note: Maximum value for Adjusted Number of Correct Folds is 25.

Table 2 .

 2 Mean Number of Correct Folds and Number of Error Folds by Spatial Ability and Orientation (standard deviations in parentheses)

		Number of Correct Folds	Number of Error Folds
		UpDown	LeftRight	UpDown	LeftRight
	High spatial	21.56 (4.50)	24.33 (0.82)	3.11 (2.93)	2.83 (2.92)
	ability	n=9	n=6	n=9	n=6
	Low spatial	15.40 (6.54)	20.88 (5.89)	8.4 (4.72)	2.88 (2.59)
	ability	n=5	n=8	n=5	n=8
	Note: Maximum value for Number of Correct Folds = 25.

Construction tasks are ubiquitous: assembling a child's bicycle, a piece of furniture or folding a paper airplane being common examples

We performed a MANOVA because intuitively, it would make sense that our dependent variables were intercorrelated in some way. The MANOVA identifies significant intercorrelations among dependent measures such that these measures are not incorrectly identified as significant effects of the independent variables. While we recognize that our test may be somewhat lacking in power, we feel that, in light of the experimental design, these statistical procedures are the most appropriate.

Acknowledgements

We thank Ron Buchanan and Chris Glenn for their assistance on this project.


Ning Zhang

Junliang Chen
email: chenjl@dlmu.edu.cn

Zhengjie Liu

Jun Zhang

Public Information System Interface Design Research

Keywords: Universal usability, Cognitive load, Public information system

The diversity of users' cognitive skills remains the challenge of public information system interface design. In this paper, we focus on the universal interaction design method for public information systems like kiosks. We have developed a method with six steps based on the resources model. The method we proposed aims at reducing users' cognitive load and enabling designers to optimize interface information. To validate this method, two prototypes were designed based on the method and a usability test was conducted to compare users' cognitive load, performance and satisfaction between the designed prototypes and the current referencing system. Results show that, in contrast with the current reference system, prototypes we designed based on the proposed method can reduce user's cognitive load, and enhance user's performance and satisfaction.

Introduction

With the widespread of public information systems (PIS) in various service industries in China, users with various backgrounds become a challenge for PIS's interface design [START_REF] Cremers | User-Centered Design with Illiterate Persons, the Case of the ATM User Interface[END_REF] . PIS designers and interface researchers have been exploring different ways to provide PIS users with more user-friendly and understandable interaction platform, including developing multi-model interface [START_REF] Mark | Adaptive User Interfaces for Public Kiosks[END_REF] , setting up accessibility guidelines [START_REF] Maguire | A Review of User-Interface Design Guidelines for Public Information Kiosk Systems[END_REF] , and providing interface solutions based on usability test [START_REF] Farooq | Haptics in Kiosks and ATMs for the Disabled[END_REF] . However, the lack of interaction design theory and design methods concerning PIS users 'cognitive levels remains the bottleneck for PIS interaction design, resulting in the loss of users and the low use rate of current PIS in China.

In this paper, a PIS interface design model based on distributed cognition theory was built to provide foundations and guide for the appropriate PIS interface design method, so that users' cognitive load can be minimized during the PIS interface design process. With hotel self-service kiosk interface design as an example, two prototypes were designed based on the proposed PIS interface design method. To validate whether PIS interface design method can reduce users' cognitive load, a comparative usability test was carried out to evaluate users' performance, satisfaction and cogni-tive load. Results confirm the validity and effectiveness of the proposed PIS interface design method.

Relevant Research

Resources model of distributed cognition theory and universal design method are main supportive research area for this study.

Resources Model

Resources model of distributed cognition theory was first introduced into humancomputer interaction (HCI) design by Wright [START_REF] Wright | Analyzing Human-Computer Interaction as Distributed Cognition: the Resources Model[END_REF] , to describe how information is distributed between users and computer systems, how to design appropriate and reasonable outer representations to minimize users' cognitive load. Resources model is mainly applied to describe human computer interaction activities and evaluate interfaces, and remains one of the most influential models in HCI field.

Resources refer to a collection of information structures that can be defined for each step in an interaction and which can be used to inform action. Information structure and interaction strategies are two components of resources model. Plan following, plan construction, goal matching and history-based selection are four strategies defined by Wright to analyze, design and evaluate interfaces. The basic information structure of resources model consists of six elements, including plan, goal, affordance, history, state and action-effect relations. Detailed definitions of the six elements are listed in figure 1. As an emerging theory in HCI, resources model is mainly applied in HCI research by Smith [START_REF] Smith | Using the Resources Model in Virtual Environment Design. King's Manor Workshop on user-centered Design and implementation of virtual environments[END_REF] , Cheng [START_REF] Cheng | Human-Computer Interaction Resource Model based on Distributed Cognition[END_REF] and Wang [START_REF] Wang | an Extended Resources Model based on Distributed Cognition[END_REF] .

Table 1. Elements definitions of information struction of resources model Element Definition

Plan A sequence of actions, events and states that could be carried out.

Goal

A required state of the system Affordance A set of possible next actions can be taken by the user for a give state of the system History Actions, events or states already achieved in the interaction

Sate

The collection of relevant values of the objects that feature in the interaction at a given point in the interaction Action-effect relation A causal relation between an action or event and state which represents the effect that executing the action or event will have on the interaction

Universal Usability

Universal usability was first put forward in interface interaction design area by Ben Schneiderman [START_REF] Shneiderman | Universal Usability as a Stimulus to Advanced Interaction Design[END_REF] , emphasizing the design of information products and services should be usable for every citizen.

Universal usability design (UUD) method includes three steps. First, it requires designers to classify different user groups that need special considerations during design process; second, build universal usability design matrix for the classified user groups, listing the specific requirements of different user groups and relevant design items; third, propose design solutions according to design matrix and evaluate the design solutions.

Since PIS users significantly differ in terms of age, education background, and other variables impacting cognitive load, it is feasible to adopt universal usability design method to guide PIS design process. As a newly introduced design method in HCI study, the application of this method in PIS interaction design is still limited, some representative exploring studies include Kouroupetroglou [START_REF] Kouroupetroglou | Universal Access in Pubic Terminals: Information Kiosks and Automated Teller Machines[END_REF] , Carbonell [START_REF] Carbonell | Universal Access: Theoretical Perspectives, Practice and Experience[END_REF] , and Takeo [START_REF] Takeo | Promoting Universal Design of Automated Teller Machines[END_REF] .

Interaction Model for PIS

There are two common ways to reduce users' cognitive load in interaction process. One is to simplify interaction flow; the other is to provide information easily understood for majority users. Resources model and UUD method can provide necessary support for the realization of the above mentioned ways. By applying resources model into the development of PIS interface, interaction flow can be simplified during design process, and cognitive resources can be allocated to systems as much as possible. Combing with UUD method, designers can provide appropriate information easily understood for majority users. Based on resources model and UUD method, PIS interface interaction model and design method can be built to help designers make reasonable interface information allocation decisions.

PIS Interface Information Structure

PIS interface differs from traditional desk interface in its full screen display, and users can only complete tasks in a certain fixed interaction flow. Besides that, public context has impact on users' interactions with PIS. Therefore, we adopt flow and context as two key elements into PIS interface information structure. Flow is defined as the set of actions during interaction process. Describing flow can help interface designers and researchers follow users' real time goals. Context is defined as users' action-related conditions and variables in human computer interaction. The illustrations of PIS interface information structure is showed in figure 1.

The PIS interface information structure in fig 1 is aimed at describing general information a user need to complete a certain task through PIS. Based on this infor-mation structure, a set of interaction strategies can be built to guide the design and evaluations of PIS interaction process.

Fig. 1. PIS interface information structure

PIS Interface Interaction Strategy

We propose three basic interaction strategies for PIS interface, including goal matching and optimization, flow construction and evaluations, and affordance identification and obtaining. These interaction strategies can describe users' possible actions and correspondent operations a system provides in interaction process.

(1) Goal matching and optimization According to goal matching and optimization strategy, users need goal, context, flow, state, and affordance to complete a task. This is a dynamic process. The state of resources changes with users' operations. For users with different cognitive skills, the matching extent of goals and resources differs. Designers have to analyze users' cognitive skills and evaluate the resources a system can provide to improve design and realize the optimized matching of users' goals and resources a system can provide. Detailed goal matching and optimization strategy is showed in figure 2. This strategy requires designers investigating users' cognitive skills before allocating resources during interface design process. Methods like card sorting and participatory design can help designers bridge the gap between users' mental model and designers' mental model, and realize goal matching in an optimum condition.

(2) Flow construction and implementation Users need information like affordance, current system state to decide next possible actions when interacting with computers. Basic operation flow of a task is composed of necessary interaction steps. Efficient and concise operation flow will facilitate the realization of goals. Detailed flow construction and implementation strategy is showed in figure 3. In figure 3, internal flow means the task completion order users perceive according to knowledge and system current state, external flow is the real task completion order of the designed system. Designers need analyze users' understanding about the system's working flow and decide the optimum flow for system development.

Fig. 3. Flow construction and implementation

This strategy requires designers building the appropriate task completion order and interface layout based on interaction task analysis. Universal usability design method can be taken into account to design interface layout. The design matrix from UUD method includes all the affordance users of different groups need to take actions according to task completion order. The set of affordance in the design matrix lists necessary points designers need consider in interface design process. PIS interface designers can make use of UUD method to propose specific design proposals for users with different background.

(3) Affordance identification and obtaining Whether or not obtaining the right affordance is crucial for users to complete an interaction task. Too much affordance will increase users' cognitive load, while the lack of affordance prevents users completing tasks. Due to the diversity of PIS users' background, different users need different affordance. Therefore, affordance identification and obtaining strategy requires designers evaluating users' cognitive load and adjusting affordance based on evaluation results. Besides interface layout, context is

Fig. 4. Affordance identification and obtaining

The above mentioned PIS interaction model reflects the relationship between actions and information presentations. However, only models are not enough for interface designers to carry out specific design. PIS interaction methods based on PIS interaction model are necessary tools for designers to operate during design process.

PIS Interaction Design Method

Goal matching and optimization strategy requires designers focusing on different user groups' cognition analysis; flow construction and implementation strategy requires designers implementing information architecture under the help of UUD method; affordance identification and obtaining strategy requires designers evaluating users' cognitive load to make sure if the design proposal is reasonable enough for users with diversified background. These strategies need to be implemented throughout the whole design process. The common user-centered design methods include user needs analysis, functional analysis, information architecture, prototype design and usability evaluation. Correspondently, the PIS interaction design method we proposed is divided into six stages, that is, users' cognitive analysis, interaction task analysis, interface information layout, design project framing, prototype design, and cognitive load evaluation. Figure 5 shows the outline of the PIS interaction design method.

Users' Cognitive Analysis

Most of PIS users are either beginners or occasionally users, named "forever middle users" by Alan Cooper [START_REF] Cooper | The Inmates are Running the Asylum: Why High-Tech Products Drive Us Crazy and How to Restore the Sanity[END_REF] , not to mention those without computer skills. Relevant research [START_REF] Cremers | User-Centered Design with Illiterate Persons, the Case of the ATM User Interface[END_REF] shows that age, education and experience are main factors influencing users' attitudes towards PIS products. Therefore, PIS users can be grouped according to their age, education and PIS product experience. After dividing users into different groups, designers need contextual inquiry and interview to analyze different users' cognition towards PIS products, watching or interviewing users about their interaction experience with PIS products, recording the problems they meet, and the reasons users' goals are not matched, including problems from contextual influence, information overload, information misleading, and information deficiency. According to ISO25062 [START_REF]Software Product Quality Requirements and Evaluation (SQuaRE)-Common Industry Format (CIF) for Usability Test Reports[END_REF] , each user group need at least 8 participants. Findings from users' cognition analysis should be edited into table form, as checklists for further evaluation reference.

Interaction Task Analysis

In interaction task analysis stage, designers need solve two problems. One is task categorization; another is to ensure task completion orders. The aim of this stage is to settle the general structure of the system being designed, and to describe the relationship between users' input and system's output.

Fig. 5. Illustrations of task completion order

Designers can categorize different tasks need to be realized in the target system by card sorting, and then build completion order for each specific task through participa-tory design method, each step of the task flow has to be numbered according to the fixed order, as it is showed in figure 5. In this way, the basic task flow of the system can be drawn. In figure 5, the work flow of the three tasks of the main menu is listed. For example, C1 in figure 5 represents the first subtask of task C, and M2 represents the second system information provided for users to complete certain task.

Interface Information Layout

Based on the results of users' cognition analysis and interaction task analysis, PIS interface design matrix can be built through UUD method. All the points listed in the matrix can help designers do specific design to reduce users' cognitive load, as it is showed in table 2. With user groups as the horizontal axis of the matrix, and task completion order as the vertical axis, designers can input the rest intersectional content of the matrix according to users' cognition analysis results, and current PIS interface design standards [START_REF] Maguire | A Review of User-Interface Design Guidelines for Public Information Kiosk Systems[END_REF] , including various specific requirements of different user groups, the solutions of their problems need to be considered. Each solution corresponds to the problem a group of user have in a certain stage of a task flow. Usually there is no appropriate data that can be used directly by designers in a specific design process, especially quantitative data. Users' self-reported data obtained by interview, observation and contextual inquiry is the main source for designing PIS interface for users with different cognitive levels.

Design Project Framing

The output of design project framing is design document. Interaction designers need to draw up specific design documents based on the design matrix, so that system developers can read in details. A basic design document is consisted of information architecture, interface layout, interface elements, task completion flow and interface order.

Prototype Design

In prototype design stage, designers can use Flash, Axure RP and other tools to realize the information visualization based on the design documents. The designed prototypes should be revised according to evaluation results. Formative evaluation methods like cognitive walkthrough and heuristic evaluation can be applied in this stage to save design and development cost.

Cognitive Load Evaluation

User's cognitive load level correlates the extent of system affordance identification and obtaining. Research has found that users' performance is not directly correlated with cognitive load level, some system are tested with high performance but low users' satisfaction [START_REF] Fisher | Why do Lay People Believe that Satisfaction and Performance are Correlated? Possible Sources of a Commonsense Theory[END_REF] . The main reason is the existence of cognitive load during human computer interaction process. Therefore, it is necessary to evaluate the cognitive load level of PIS users.

Tapping test proves an effective and efficient method to test users' cognitive load and system's usability [START_REF] Albers | Tapping as a Measure of Cognitive Load and Website Usability[END_REF] . Comparison test is also necessary for designers to compare users cognitive load levels when interacting with different prototypes. Cognitive load can be evaluated by NASA-TLX [START_REF] Hart | NASA-Task Load Index (NASA-TLX):20 Years Later[END_REF] , Paas Scale [START_REF] Paas | Cognitive Load Measurements as Means to Advance Cognitive Load theory[END_REF] , and user performance.

Case Study

In order to test the validity and feasibility of the proposed PIS interaction method, we recruited two interaction designers to design hotel self-service system interface according to the requirements of PIS interaction design method. We chose PIS in hotel industry because of the increasing tendency of PIS applications in hotels, and the diversity of hotel clients. Prototype system A and B designed by PIS interaction design method are showed in figure 6 and figure 7. We compare the prototypes with a current running hotel selfservice system in Shandong, China, as the reference system showed in figure 8. Proto-type systems and the reference system will be compared in terms of users' cognitive load, performance and satisfaction.

The comparison test is conducted in the usability lab of Dalian Maritime University, China. We recruited 32 users to participate in our test, and divide these users in two groups. One is mainstream user group, with 16 users using PIS at least once a month, aged from 19 to 28, all the users in mainstream user group are university students; the other is non-mainstream user group, with 16 users no PIS product experience, aged from 45to 78, all the users in testing group have no bachelor degree.

Fig. 8. Reference System

All users are required to complete three tasks on touchscreens, including selfservice check-in, self-service check-out, and room renewal, using prototype system A and B, as well as reference system, When interacting with systems, users are asked to tap with the rhythm of the music playing in the usability lab. Users' interaction process is recorded to calculate task completion time and error rate. Users are also asked to complete NASA-TLX scale and SUS (system usability scale) to calculate their cognitive load and satisfactions towards different systems. Test results are listed below. Figure 9 shows that the cognitive load of the mainstream user group is lower than the cognitive load of the testing group when they complete the designed tasks through the three systems. Both of the groups' cognitive load is lower when using prototype systems than using the reference system.

Fig. 10. Comparison of satisfaction means

Figure 10 shows that the mainstream users' satisfaction towards the three systems is higher than non-mainstream group's satisfaction. Both of the groups' satisfaction towards prototype systems is higher than towards reference system. Tapping test results show that the ratios of mainstream users and non-mainstream users' missing the rhythm of the music when completing the designed task through the three systems are8:6, 4:3, and 1:1 respectively, as it is showed in figure 12.

Fig. 12. Tapping missing times

Independent T test results show that there is significant difference between users' satisfaction towards reference system and towards prototype systems. There is also significant difference between users' performance on the reference system and on the prototype systems. However, there is no significant difference between users' cognitive load on the reference system and on prototype systems, and this result needs further validation among broader and more representative sample.

Data analysis results mentioned above, to some extent, support the validity and feasibility of the PIS interaction method.

Limitations and Recommendations

Due to the limitations of the research sample, the results of the study still need further validation. The comparison test was conducted in usability lab, although tapping test was applied to simulate the real interaction environment, there is still difference between lab environment and the real world situation.

Fig. 2 .

 2 Fig. 2. Goal matching and optimization strategy

  factor influence users' affordance identification and obtaining. If users' cognitive load is tested in usability lab, designers can use tapping test to simulate real operation situations, and to enhance the reliability of evaluation results.

Fig. 6 .

 6 Fig. 6. Prototype A Fig. 7. Prototype B

Fig. 9 .

 9 Fig. 9.Comparison of cognitive load means

Fig. 11 .

 11 Fig. 11. Comparison of task completion time means

Table 2 .

 2 Examples of design matrix

	user
	task	Senior users	Fresh users	Low educated users
	Step 1	××	××	××
	Step 2	××	××	××
	Step 3	××	××	××
	:	××	××	××
	:	××	××	××
	Step n	××	××	××