Matthias Peissner
email: matthias.peissner@iao.fraunhofer.de

Rob Edlin -White

Rob Edlin-White

User Control in Adaptive User Interfaces for Accessibility

Keywords: Adaptive user interfaces, design patterns, accessibility, user study, controllability

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

Introduction

Self-learning and self-adaptive user interfaces offer big potential for accessible technology. In contrast to one universal design for all, personalized user interfaces can offer optimal levels of usability and accessibility for diverse users and context conditions by individual design solutions [START_REF] Weld | Automatically personalizing user interfaces[END_REF]. For personalization in accessible technologies there are two main approaches: adaptable and adaptive user interfaces (cf. [START_REF] Findlater | A comparison of static, adaptive, and adaptable menus[END_REF]). Most adaptable user interfaces require complex configuration dialogues which can cause problems for the users [START_REF] Mackay | Triggers and barriers to customizing software[END_REF] -especially for those who would benefit most from personalization. Therefore, adaptive user interfaces can be more effective in increasing accessibility. They include self-learning mechanisms to identify individual needs and system-initiated user interface adaptations to avoid and overcome barriers of use automatically and directly when they occur. However, self-adaptive user interfaces also pose significant challenges in terms of usability, trust-worthiness and acceptability. The most prominent issues include con-fusing inconsistencies and the feeling of losing control over automatic changes in the user interface [START_REF] Jameson | Understanding and Dealing with Usability Side Effects of Intelligent Processing[END_REF] [START_REF] Weld | Automatically personalizing user interfaces[END_REF]. Hence, mechanisms to improve the transparency and controllability of automatic adaptations will be a necessary feature of successful adaptive user interfaces. In this work, transparency means that users will be able to recognize and understand the automatic changes in the user interface and that confusion and disorientation will be avoided by making clear what has been changed and by supporting a "feeling of continuity between the user interface before and after adaptation" [START_REF] Dessart | Showing user interface adaptivity by animated transitions[END_REF]. Moreover, it can be useful to explain the reasons for adaptations to the user in order to increase the predictability of the system behavior [START_REF] Jameson | Understanding and Dealing with Usability Side Effects of Intelligent Processing[END_REF]. Adaptive user interfaces are said to be controllable if the user can control the intelligent and automatic adaptations. This can include opportunities for the user to prevent or actively accept adaptations, and to undo or override adaptations (cf. [START_REF] Wobbrock | Ability-based design: Concept, principles and examples[END_REF]). Many authors (e.g. [START_REF] Norman | How might people interact with agents[END_REF] [START_REF] Vanderdonckt | Multimodality for Plastic User Interfaces: Models, Methods, and Principles[END_REF]) consider full user control over automatic adaptations as a major requirement of acceptable adaptive systems. For adaptive systems which aim at improving the accessibility, however, this claim might require reinterpretation. Impaired users sometimes need automatic adaptations in order to be able to access certain functionality and to control the interface at all. Trewin [START_REF] Trewin | Automating accessibility: the dynamic keyboard[END_REF] argues that these users might experience an even increased rather than decreased level of overall control with automatic adaptations which require no extra interaction effort.

Adaptation mechanisms are error prone. Especially, incorrect adaptations challenge the transparency and controllability of an adaptive system. They lead to undesired user interface changes and make it difficult for the user to predict and understand the system behavior (cf. [START_REF] Findlater | Design Space and Evaluation Challenges of Adaptive Graphical User Interfaces[END_REF] [START_REF] Jameson | Understanding and Dealing with Usability Side Effects of Intelligent Processing[END_REF]). Weld et al. [START_REF] Weld | Automatically personalizing user interfaces[END_REF] point out that incorrect adaptations are associated with costs for the user which have to be taken into account together with the potential advantages of an adaptive system. Systematic cost-benefit considerations cover also the costs of correct adaptations such as becoming aware of and leveraging the adaptation [START_REF] Gajos | Exploring the design space for adaptive graphical user interfaces[END_REF]. As a consequence, adaptive user interfaces will be successful only if the subjective benefits of the adaptations outweigh their costs [START_REF] Findlater | Design Space and Evaluation Challenges of Adaptive Graphical User Interfaces[END_REF]. Intelligent technologies will be limited in their ability to optimize individual cost-benefit ratios. Mechanisms to put the user into control of the system's adaptation behavior can be regarded as an effective means to optimize the subjective utility of automatic adaptations.

We recognize that the design of the adaptation process is a major issue for the usability and acceptability of adaptive user interfaces. In our MyUI system, we address this important field with dedicated Adaptation Patterns. They inform the users about intended or current adaptations. And they provide the users with dialogue mechanisms to optimize the costs and benefits of automatic adaptations by increasing their transparency and controllability [START_REF] Peissner | MyUI: Generating accessible user interfaces from multimodal design patterns[END_REF]. This paper concentrates on an in-depth examination of the effectiveness and acceptability of the MyUI adaptation patterns from the users' perspective. This work makes the following contributions:

1. We introduce a systematic cost-benefit analysis of user interface adaptations for accessibility which serves as a basis for the experimental evaluation of adaptation dialogue mechanisms under different conditions.

2. The results of our study provide valuable insights into the interactions between the costs and benefits of an adaptation and the preferred level of user control. We discuss how the findings can be used for the design of effective adaptation dialogues.

Related Work

The preconditions and problems of transparency in adaptive user interfaces are well described in the literature. Findlater and Gajos [START_REF] Findlater | Design Space and Evaluation Challenges of Adaptive Graphical User Interfaces[END_REF] report that adaptive menus are more understandable and predictable if they preserve spatial stability and if the adaptations are "elective", i.e. they can be ignored by the users. Tsandilas and Schraefel [START_REF] Tsandilas | An Empirical Assessment of Adaptation Techniques[END_REF] point out the negative effect of poor adaptation accuracy on user performance in adaptive menus. They explain their findings by a decrease of user reliance on the system and thereby a decrease of subjective predictability of the system's behavior. However, only few approaches to increase the transparency of automatic adaptations have been published. In a first systematic attempt, Dessart et al. [START_REF] Dessart | Showing user interface adaptivity by animated transitions[END_REF] propose animated transitions for showing the adaptation process to the user. They develop a catalogue of "adaptation operations" (e.g. resizing, relocating, etc.) and suggest suitable animated transitions to support continuity in the perception of the dynamically changing user interface. Other approaches aim at a deeper user understanding of the system's adaptations by providing detailed explanations [START_REF] Bunt | A Mixed-Initiative Approach to Interface Personalization[END_REF] or "by supporting the user in developing an adequate mental model of the systems' adaptation mechanisms" [START_REF] Kühme | A user-centered approach to adaptive interfaces[END_REF]. However, it seems questionable if and how these approaches can be applied to adaptations which try to compensate for perceptual, cognitive and motor impairments in users.

The problem of controllability has mainly been treated in terms of theoretic taxonomies rather than specific design guidelines. In the concept of PLASTIC USER INTERFACES, a "supra-UI" [START_REF] Calvary | Envisioning Advanced User Interfaces for e-Government Applications: a Case Study[END_REF] or "meta-UI" [START_REF] Coutaz | User interface plasticity: model driven engineering to the limit![END_REF] is envisaged by which the user can control the system's adaptations. Three levels of controllability are distinguished: At the lowest level, the user can only observe but not influence the adaptation process. At the intermediate level ("approbation"), the user can accept or reject an adaptation proposed by the system. The highest control level is characterized by a full user control over the adaptation ("specification") [START_REF] Calvary | Envisioning Advanced User Interfaces for e-Government Applications: a Case Study[END_REF]. With Jameson's [START_REF] Jameson | Understanding and Dealing with Usability Side Effects of Intelligent Processing[END_REF] suggestion to collect simple user feedback after performing an automatic adaptation (e.g., "I don't like what just happened"), the above mentioned approbation strategy can be further subdivided into a confirmation before and a confirmation after the adaptation.

Lavie and Meyer [START_REF] Lavie | Benefits and costs of adaptive user interfaces[END_REF] compare four similar adaptation strategies with different levels of user control in a user study: a "manual" condition, in which the user performs all actions; "user selection" where the user selects one of the alternative adaptations suggested by the system; "user approval" where the user can accept or reject the system-initiated action; and "fully adaptive" where the system performs all actions and the user can just abort the adaptation process. The four strategies differ in their effectiveness in supporting a secondary task during a driving simulation session. In routine situations where the system can offer correct adaptations, the fully adaptive variant is the most beneficial for the users. In unfamiliar situations with inappropriate system adaptations, the other conditions with active user involvement perform better. This cost-benefit pattern is even more apparent in their second study with older participants. They benefit more from automatic adaptations when they are correct -but experience higher costs in cases of incorrect adaptations [START_REF] Lavie | Benefits and costs of adaptive user interfaces[END_REF]. Even if Lavie and Meyer's experiment investigates adaptivity in the sense of automating the performance of a user task, their results can be easily transferred to adaptive user interfaces for improved accessibility. For users with more difficulties in using technology, the benefits of correct adaptations seem to clearly outweigh the costs of losing control over the system's adaptation behavior.

Another valuable contribution of Lavie and Meyer's work is that they analyze the utility of adaptations in combination with different control mechanisms. Earlier research has focused on the costs and benefits of different adaptations [START_REF] Horvitz | Principles of mixed-initiative user interfaces[END_REF] [13] [9] [START_REF] Findlater | Design Space and Evaluation Challenges of Adaptive Graphical User Interfaces[END_REF]. But Lavie and Meyer [START_REF] Lavie | Benefits and costs of adaptive user interfaces[END_REF] investigate how different adaptation control mechanisms can influence the costs and benefits of the same adaptations.

However, for a systematic analysis of the costs and benefits of adaptations and their interactions with adaptation control mechanisms, their study has two main shortcomings. Firstly, the specific influence of costs and benefits cannot be separated since only two of the four possible cost-benefit conditions (one with high benefit and one with high cost) have been compared. Secondly, only objective performance measures have been taken. However, the predominant disadvantages of adaptive systems as reported in the literature do not relate to performance but to more subjective concepts such as experienced transparency and the feeling of control.

3

Adaptations in the MyUI System

Overview of the MyUI system

This section summarizes the main features of the MyUI system (see [START_REF] Peissner | MyUI: Generating accessible user interfaces from multimodal design patterns[END_REF] for a more detailed description) to provide the conceptual background for the experimental user study. MyUI aims at mainstreaming accessible and highly individualised ICT products. Instead of developing dedicated user interfaces for specific user groups with certain disabilities, MyUI tackles the problem by providing a generic infrastructure to enhance mainstream products with automatically generated user interfaces which adapt to diverse user needs, different devices, and changing context conditions. MyUI adaptations include changes in the presentation formats and modalities, interaction mechanisms and navigation paths. MyUI takes a modular approach to adaptive user interfaces relying on the composition of multimodal user interface design patterns. Individual accessibility is achieved by combining patterns which provide proven solutions for specific interaction situations and characteristics of the user, environment and device [START_REF] Peissner | MyUI: Generating accessible user interfaces from multimodal design patterns[END_REF]. MyUI strives for a smooth and natural adaptation process by avoiding the need of an initial configuration or user enrolment process. The MyUI system senses and interprets information about the user and the context of use directly during the interaction and refines the profiles accordingly. Profile changes immediately lead to automatic run-time adaptations of the user interface. This closed loop of self-learning user and context profiles and system-initiated run-time adaptations supports a continuously improved fit between the adaptive user interface and individual user needs. Thus, also altering user capabilities as typical in the course of aging and rehabilitation (cf. [START_REF] Sloan | The potential of adaptive interfaces as an accessibility aid for older web users[END_REF]) can be covered. Moreover, barriers of use can be overcome directly when they occur.

Profile changes that lead to user interface adaptations are triggered by a number of sensor events. Most triggering events are detected from the user's interaction behaviour with the current user interface. Examples include time-outs (the user does not react to a system prompt within a certain time frame), repeated undos (the user seems to repeatedly select wrong options by accident) and detours. Moreover, also hardware-based sensors as, for example, eye tracking in order to capture the user's attention can be used in the MyUI user profiling process (cf. [START_REF] Hernández | Prototype for user context management infrastructure and user modelling[END_REF]). In order to avoid too many changes on the user interface and to compensate for the uncertainty of the sensor-based recognition procedures, user profiling involves statistical processing mechanisms. As a result, not every single sensor event will immediately trigger an adaptation, but adaptations will be triggered only if a certain threshold in the user profile is exceeded. This supports user interface stability. But on the other hand, transparency suffers. Users can have problems in understanding and anticipating automatic adaptations. It can be difficult to establish a clear connection between the adaptations and their causes, i.e. the triggering events.

MyUI adaptation patterns

The MyUI design patterns repository [START_REF]MyUI Design Patterns Repository[END_REF] includes different types of patterns which serve distinct functions in the adaptation process. There are patterns for creating and updating a user interface profile which includes global variables to define general settings throughout the entire user interface (e.g. font size). Other patterns provide alternative user interface components and elements for current interaction situations. Finally, adaptation patterns describe the transition from one instance of the user interface to another. Their main goal is to assure high levels of transparency and controllability for the user during automatic adaptations. There are two types of adaptation patterns:

• Adaptation rendering patterns specify the transitions from the user interface before the adaptation to the user interface after the adaptation. They use animations as recommended by [START_REF] Dessart | Showing user interface adaptivity by animated transitions[END_REF] to create continuity in the course of adaptations. The animated transitions draw the user's attention to the changing screen areas and make clear the relationship between the new, adapted user interface and the former user interface. Examples include animations to grow small fonts to bigger fonts or to move elements from one area to another. Adaptation rendering patterns are always part of an adaptation process as specified by the adaptation dialogue patterns. • Adaptation dialogue patterns specify the dialogue around an adaptation to make sure that the user is aware of the adaptation and can control the system's adaptation behaviour. Typical elements of an adaptation dialogue include a notification and interaction options for the user to control the system's adaptation behaviour. The MyUI patterns repository includes different adaptation dialogue patterns with different levels of user control.

The main objective of this paper is to investigate the effectiveness and acceptability of the different adaptation dialogue patterns in different cost-benefit situations and to understand their contribution to optimize the subjective costs and benefits of an adaptation as experienced by the user. Before presenting the experimental design and the results of our user study, we describe the patterns and their assumed effects on the usability and acceptability of automatic run-time adaptations.

Adaptation dialogue patterns

Automatic adaptation without adaptation dialogue (baseline). This pattern can be regarded as a baseline condition. It is currently not used in the MyUI system. The adaptation is triggered and performed automatically. Besides the animated transitions of the involved adaptation rendering patterns, there is no additional indication of the current adaptation. The user has no opportunity to control, cancel or undo the adaptation.

Automatic adaptation with implicit confirmation (AI).

During the automatically triggered adaptation, an animated icon notifies the user about the on-going adaptation in a dedicated adaptation area on the screen. When the adaptation is finished the end user can undo the adaptation via a button in the adaptation area. Moreover, the adaptation area offers a permanent access to the user interface profile and the user profile.

Compared to the baseline condition, this pattern is expected to slightly increase the transparency and controllability by adding the animated icon and the undo opportunity. However, both mechanisms are unobtrusive and require a certain level of system experience and attention. On the other hand, this type of adaptation control will not decrease the interaction efficiency as no additional steps are required if the user agrees with the proposed adaptation.

Explicit confirmation before adaptation (EB).

Before the user interface is adapted, a dialogue box is displayed which requests the user to explicitly accept or reject the adaptation. A preview of the user interface after the adaptation supports the user's decision. If the user rejects the adaptation, the dialogue box is closed and the current user interface is not changed. If the user accepts the adaptation or if the system receives no user input (time-out), the dialogue box is closed and the adaptation is carried out.

This pattern is assumed to strongly increase the transparency and controllability. The dialogue box offers an obvious hint to the planned adaptation and does not start the adaptation before the user provides an explicit agreement. However, it interrupts the current interaction and requires an extra step in the dialogue. This might cause problems for some users and reduce the efficiency -especially with adaptations which are perceived beneficial by the user.

Explicit confirmation after adaptation (EA).

When the automatically triggered adaptation has been finished, a dialogue box asks the user if the changes shall be kept or undone. In case of rejection, the adaptation is recalled. In case of acceptance or a time-out event, the adaptation is kept.

This pattern is similar to the EB pattern. We expect that due to the delayed confirmation dialogue, the transparency and controllability gains will not be as high as with the EB pattern. It also requires the described extra step.

4

Experimental User Study

Goals

Our study had two main goals. First, we wanted to validate the MyUI adaptation dialogue patterns and evaluate their effectiveness and acceptability compared to the baseline condition. Second, we wanted to gain insights in how to implement the patterns in the MyUI adaptation engine. Especially, we wanted to know if we can use a simple adaptation process with only one design pattern which is preferable under all conditions or if more complex adaptation process behaviour will be required in order to present different patterns in different cost-benefit conditions and to different users.

Hypotheses

User interface adaptations can support the accessibility by detecting and overcoming specific problems or barriers of use during the interaction. In these cases, adaptations will provide a benefit for the user. However, wrong system assumptions about individual user requirements can lead to adaptations which are not beneficial for the user.

On the other hand, adaptations can also cause costs by raising the effort for task completion. This leads us to the distinction of adaptations with low vs. high costs and benefits:

• Adaptations with high benefits help to overcome an individual interaction barrier, e.g. increase the font size when the user has vision problems. • Adaptations with low benefits have no positive usability effect for the user in the current situation, i.e. the user does not experience a barrier of use before the adaptation occurs. This condition represents useless adaptations as an effect of user profiling errors. • Adaptations with high costs require an extra interaction effort compared to the user interface before adaptation, e.g. after increasing the font size, the user needs to scroll down the page in order to access the desired information. • Adaptations with low costs do not require extra interaction effort.

The four cost-benefit conditions which result from a 2x2 combination of low and high costs and benefits are systematically produced in our experiment and serve as the experimental context conditions for testing the above described adaptation dialogue patterns. For each pair of pattern and cost-benefit condition, subjective user assess-ments are recorded to evaluate the perceived transparency, controllability, comfort of use and acceptance of the patterns as well as the subjective costs and benefits of the adaptations.

The following four research hypotheses are investigated:

1. The adaptation patterns AI, EB and EA perform better than the baseline in terms of subjective transparency, controllability, as well as acceptability and user preference. 2. The adaptation patterns differ in their capability to support users in optimizing the subjective utility of an adaptation. We assume that control mechanisms can help users to minimize the subjective costs of undesired adaptations. 3. In different cost-benefit conditions, users prefer different adaptation patterns and the patterns receive different levels of user acceptance. We assume that high-cost adaptations are associated with a stronger wish for control while high-benefit adaptations will lead to preferring more efficient patterns. 4. User preference and acceptance of the patterns differ for different user groups, e.g. users with higher levels of technology anxiety prefer patterns with more control.

Methods

Participants. For this experiment, twelve older adults were recruited. They were paid € 50 to participate for one test session of about 90 minutes. The participants were between the ages of 49 and 73, with an average of 63 years; there were seven females and five males. None of them reported significant accessibility problems. Three had minor reading problems with small fonts, two said they preferred bigger keys on remote controls, and four reported problems with the understandability of instructions or technical terms in interactive products. Six described their ICT literacy level as medium, three as low, and three as fairly good.

The selection of able-bodies participants for this experiment was deliberate. In accordance with ISO 9241-171 [START_REF]2008 Ergonomics of human-system interaction -Part 171: Guidance on software accessibility[END_REF], accessibility does not only apply to disabled users but also to users with temporary impairments or users who experience difficulties in particular situations, etc. Common to all users who benefit from an adaptive and accessible system like MyUI is the experience of a barrier which is then removed by an adaptation -regardless of the many different possible reasons and disabilities. All our users were at ages where barriers of use can arise quite suddenly, even without disabilities. Involving only disabled users might cause a bias as these users might have found their ways to cope with accessibility issues and they might be prejudiced against intelligent solutions from prior experiences. Moreover, our able-bodied sample helped us to exclude undesired side effects of disabilities which are not covered by the design solutions presented in the experiment. Working with participants without disabilities allows us to fully control the cost-benefit conditions.

Thus, a major assumption of our study is that preference and acceptance of the adaptation patterns will not depend on a specific type or level of impairment but mainly on the experienced costs and benefits of the offered adaptation.

Apparatus. The experiment was conducted in a typical usability lab setting. Three Windows-based computers were used. The first computer ran the interactive test prototype which simulated an adaptive interactive TV system with a main menu, and an email application to receive, view, compose and send emails. This computer was connected to a 32" PHILIPS TV screen (32PFL7605H 12) and was controlled by an infrared remote control as commonly used with TV devices. The other two computers were used for accessing and manipulating the MyUI user and context management infrastructure to start adaptations by changing user profile variables in a Wizard-of-Oz manner. The participants were sitting on a sofa in front of the TV screen with a viewing distance of about 2 meters. They had to work on different tasks with the prototype system while using the remote control.

Tasks, adaptations and cost-benefit conditions. The following three test tasks were used. During each task, a system-initiated adaptation was performed:

1. Task: Select an application from the iTV main menu.

Adaptation: Add numeric key labels to the menu items in order to allow for direct selection via numeric keys instead of cursor navigation. This adaptation is expected to help motor impaired users with difficulties in precise and repeated key presses. 2. Task: Select an email from the email inbox.

Adaptation: Replace sender names by photos. This adaptation shall help cognitively impaired user who have difficulties in language reception or text reading. 3. Task: Read a received email message.

Adaptation: Increase font size. This adaptation helps users with low vision.

With these pairs of tasks and adaptations, the four cost-benefit conditions were implemented. While the costs could be easily manipulated by the task instructions and the user interface design, the benefit conditions required special attention. The older participants were deliberately selected to have no considerable accessibility problems, i.e. none of the adaptations above would usually be beneficial for them. In order to systematically vary the benefit level we artificially induced interaction barriers (figure 1) which could be removed by user interface adaptations. In the first task (menu selection), the high-benefit condition was created by instructing the users to wear leather gloves in size XL which should pose a significant barrier to key pressing on the remote control. As a beneficial adaptation in this condition, enabling direct numeric key selection reduced the required number of key presses. In a similar manner, the Windows font type "Wingdings" was used for all text elements in the second task (inbox) so that the text output did not make any sense for the participants. In the high-benefit condition, this artificial barrier was overcome by adding sender photos to the email items so that the participants could easily recognize the searched email. In the third task (read email), glasses with dioptre values of +1.5 and +2.5 were used to pose reading problems which could be alleviated by increasing the font size adaptations in the high-benefit condition. For the example of the task "read email", figure 2 shows how the four cost-benefit conditions were produced by different instructions and artificial barriers of use.

It is important to explain that the artificial barriers (gloves, nonsense font type and glasses) were not intended to simulate any motor, cognitive or perceptual impairment in a realistic way. It would not be reasonable to assume that wearing gloves, for example, could create an interaction experience which is similar to the experience of a user with impaired finger precision. Moreover, we do not claim that this approach could cover the entire area of potential accessibility problems in a representative manner. However, it was our goal to systematically vary the benefits of automatic adaptations for accessibility. Therefore, it was important to produce the general underlying mechanisms of beneficial adaptations: The user experiences an interaction barrier and is not able to complete a given task. Then a system-initiated adaptation occurs and helps the user to overcome the problem. In order to prove the validity of our approach, we asked the participants to rate the subjective benefit of each experienced adaptation. We yielded a substantial level of agreement between the intended benefit condition and the subjective benefit ratings (Cohen's Kappa κ=0.729, n=96). The experimental variation of the costs of adaptations is supported by a fair agreement of experimental conditions and user ratings (Cohen's Kappa κ=0.375, n=96). Experimental design and measures. The study was a 4 (cost-benefit condition) x 4 (adaptation dialogue patterns) within-subjects design. Task and adaptation type was not used as another independent variable but was varied systematically to make sure that our findings were not restricted to only one type of task and adaptation and that they could be generalized. Each of the three task/adaptation conditions was performed twice in all 4 x 4 test cases. The total of 96 test conditions was randomly assigned to the twelve participants so that each participant performed eight tasks. And each participant experienced two of the three task/adaptation conditions for four times, each with all four cost-benefit conditions and all four adaptation dialogue patterns -the first task/adaptation condition naïve and the second set of four runs after they were introduced into the different adaptation patterns. The restriction to two of the three task types was due to time limitations. In a pre-study we found that eight tasks were the possible maximum within the intended time frame of 90 minutes per session.

The dependent variables of the study included subjective assessments of the adaptation patterns after each of the eight test cases per participant on a four-point Likert scale for:

• Transparency (two items: "The design is good in communicating that a change is being processed" and "The design is good in communicating what the user can do in order to stop or undo the change") • Controllability (two items: "I feel in full control of the system, its appearance and changes" and "It is easy to control the system and its appearance and changes.")

• Comfort of use (two items: "The design is good in supporting a comfortable system use" and "The design is good in avoiding unnecessary user input or interaction steps.") • General acceptance (one item: "All in all, I like the presented style of dialogue when the system is adapting to my individual needs.")

Moreover, the participants assessed the costs and benefits of the eight experienced adaptations on four-point Likert scales. Finally, the participants indicated their preferred pattern for each of the four latter test cases and a general preference in the posttask interview at the end of the session.

Procedure. The overall procedure of a single test session was as follows:

1. After a welcome and introduction to the test situation, a questionnaire was used to obtain information on the user's ICT literacy, technology interest and technology anxiety. 2. The participants had to complete a sequence of four interaction tasks with the interactive TV application. During each interaction, an adaptation occurred with a different hypothesized cost-benefit level and a different adaptation dialogue pattern. In the high-benefit condition, the adaptation was triggered (Wizard of Oz) after a time-out of 10 seconds when the participant was stuck with the artificial barrier. In the low benefit condition, the adaptation was triggered 20 seconds after the participant has started to work on the task with no obvious triggering event. After each task the participants assessed the transparency, controllability, comfort of use and overall acceptance of the adaptation pattern and the subjective costs and benefits of the adaptation via a questionnaire. 3. After the first four tasks, the participants were interviewed about the experienced adaptation patterns. If they did not recognize the differences, the test facilitator explained the different adaptation dialogue patterns. 4. The participants worked on another sequence of four interaction tasks with different intended cost-benefit conditions and different adaptation patterns. Again the questionnaire for the subjective assessment was filled after each task and the participants were asked to indicate which of the four different patterns they would prefer in the current situation. 5. A final interview was conducted to discuss general preferences regarding the patterns and to collect qualitative user feedback.

Results

Transparency, controllability, comfort of use and acceptance. The adaptation dialogue patterns AI, EB and EA were rated significantly better than the baseline adaptation in terms of transparency, controllability and acceptability (figure 3). Data analysis referred to the number of positive assessments of a criterion. For a two-item concept, this requires both items assessed positive (3 or 4 on the four-point Likert scale). A chi-square test against the H0 of equally distributed user ratings for all four patterns revealed significant differences for transparency (χ 2 (3, N=96) = 19.80, p<0.001**), controllability (χ 2 (3, N=96) = 12.42, p=0.006**) and acceptability (χ 2 (3, N=96) = 8.38, p=0.039*). As assumed, the dialogue pattern EB yielded the best assessments for transparency and controllability. The comfort-of-use ratings did not differ between the four patterns. The analysis of the user preferences yielded similar results (table 1). The patterns differed significantly in their user preference for the single test tasks (χ 2 (3, N=48) = 11.00, p=0.012*). In only 3 out of 48 test cases (6.25%), the baseline condition was the preferred adaptation mechanism. The patterns AI and EB were preferred by most users. Control of subjective costs of adaptations. Considering all 96 test cases, the adaptation dialogue patterns did not differ in their capability to reduce the subjective costs (Cost subj. ) compared to the costs as intended by the experimental condition (Cost int. ). The number of cases where participants reported low subjective costs while being in a high-cost test condition, were equal for all four patterns (χ 2 (3, N=96) = 0.54, p=0.91).

However, when users were aware of the different adaptation patterns after the review, significant differences were found (χ 2 (3, N=48) = 8.76, p=0.033*). According to our assumptions, the two patterns with explicit confirmations seemed to be most effective in reducing the costs of high-cost adaptations (table 2).

For the subjective benefit ratings, no effects of the patterns were found. It seems that even patterns which require extra effort do not reduce the advantages of adaptations which are beneficial for the users. Preference and acceptance depend on cost-benefit conditions. In contrast to our assumption, user preferences did not differ between different cost conditions. A reason might be that already in the low-cost conditions, the patterns with high control levels were preferred very often. A further effect into the expected direction might have been made more difficult by a ceiling effect. However, a significant interdependency of the benefits as intended by the experimental conditions and the pattern preferences was discovered (χ 2 (3, N=48) = 8.07, p=0.045*) (table 3). Especially, the implicit confirmation pattern AI was more often preferred with highly beneficial adaptations. This effect was even stronger for the subjective benefit as reported by the users (χ 2 =15.70, df=3, p=0.001**). As predicted, when the benefits of an adaptation were high enough, controllability loses its importance and the more efficient implicit confirmation (AI) was preferred.

The different cost and benefit conditions had no significant effect on the acceptance ratings of the different patterns, when regarding the intended utility. However, subjective costs and acceptance judgments were significantly associated (χ2 (7, N=96) = 22.21, p=0.002**). Especially, the baseline pattern and EA received less positive acceptance judgments when the adaptation caused higher subjective costs. This result indicates that adaptation mechanisms with insufficient user control will be disliked when adaptations with high subjective costs occur. On the other hand, acceptance judgments were also significantly associated with the subjective benefits of the adaptation (χ2 (7, N=96) = 14.38, p=0.045*). Especially, the baseline pattern gained higher acceptance with high-benefit adaptations. This finding is in line with the above reported result of less important control mechanisms for beneficial adaptations. Influence of user characteristics. The collected measures of individual ICT literacy, technology interest and technology anxiety were not associated with the users' acceptance and assessment of the patterns, except for the comfort-of-use judgements were significant interdependencies with ICT literacy (χ 2 (7, N=96) = 24.73, p<0.001**) and technology interest (χ 2 (7, N=96) = 15.02, p=0.036*) were found. Users with higher levels of ICT literacy rated the comfort of use of EB better whereas users with low ICT literacy rated the comfort of use of EA better. Finally, users with low technology interest judge the comfort of use of EA better than users who are more interested in technology.

Discussion

Our work provides several interesting insights with respect to control mechanisms for automatic user interface adaptations. Some of our initial assumptions could not be confirmed by the empirical results. The rigid nonparametric statistical analyses might have been responsible for a relatively low statistical power. Our main findings can be summarized as follows:

The two adaptation dialogue patterns AI and EB have clearly shown to be best in terms of transparency and controllability. It seems that an optimal adaptive system will need both approaches: a quick and efficient adaptation process which provides only an undo for unsuccessful adaptations and a more obvious and explicit dialogue which waits for a user confirmation before changes are executed. The two preferred patterns are quite different and have their specific advantages under different conditions.

The explicit confirmation pattern proved to be more appropriate for situations in which adaptations are likely to cause costs for the user. To a certain degree, it seems that explicit control mechanism can reduce the negative impressions of automatic adaptations. In our experiment, the users still had to face the same inconveniences. But the fact that they can decide about the system's behaviour seems to alleviate the subjective disadvantages. On the other hand, the extra interaction step needed for the explicit conformation did not reduce the advantages of a helpful adaptation.

However, in situations where adaptations provide great benefits to the user, the implicit adaptation dialogue (AI) was clearly preferred. In this regard, our experiment confirms Trewin's [START_REF] Trewin | Automating accessibility: the dynamic keyboard[END_REF] claim of a minor role for user control in adaptive user interfaces for accessibility. Even our baseline pattern with poor overall ratings for transparency and controllability was significantly better accepted when the users experienced a beneficial adaptation.

Our analysis of certain user characteristics and their influence on the subjective assessments of the different patterns brought the EA pattern into the discussion again. The three tested characteristics ICT literacy, technology interest and technology anxiety are covered by the MyUI user profile. It would be very easy to present the EA pattern to users with low levels of ICT literacy and technology interest. But the reported interaction effect is restricted to the EA pattern. And its absolute rating levels are -even for the most positive cases -still not better than for the other two adaptation dialogue patterns AI and EB.

Therefore, a combination of the two preferred adaptation dialogue patterns seems most appropriate. Based on these findings, we will refine the MyUI adaptation engine in a way that adaptations which cause only minor changes (i.e. changes without switching to other interaction design patterns, cf. [START_REF] Peissner | MyUI: Generating accessible user interfaces from multimodal design patterns[END_REF]) will be associated with the implicit design pattern AI. Other adaptations which cause more substantial changes, e.g. substituting one user interface component by another, will engage the explicit adaptation dialogue pattern EB.

We have implemented a novel experimentation approach which aimed at producing generalizable results. Our cost-benefit classification aimed at generalizing from specific user interface adjustments and our procedure of systematically varying the adaptations' costs and benefits seemed to be effective in generalizing from specific impairments. If our assumptions are valid, we can generalize the findings to the design of control mechanisms for adaptive systems in general.

However, if the preference of control mechanisms depended more on specific levels and types of impairments rather than the experienced costs and benefits of an adaptation, a study with actually impaired user would be indispensable. Another limitation of our study concerns the fact that we have explained the different adaptations to the users after the first half of the experiment. This might have biased attention and awareness for the adaptation process. It is not clear how the findings apply to the real world where some users might never pay attention to the way adaptations are carried out. Finally, we were mainly interested in the preconditions of user acceptance for adaptive user interfaces. Therefore, we concentrated on aspects of the user experience and subjective ratings. In order to gain a wider view on the topic, it might be interesting to also collect objective data, e.g., about improvements in task performance for the different adaptation patterns.

In order to further validate this work we suggest a similar study with a less controlled but more realistic setting. This includes involving users with actual accessibil-ity issues and presenting adaptations which specifically fit their individual needs. This future study shall also provide the opportunity to investigate if similar results can be achieved when the users experience the adaptation behaviour over a longer period of time without getting explicit explanations about the adaptation patterns.

Conclusion

The MyUI adaptation dialogue patterns mark a valuable approach to improve the transparency and controllability of automatic run-time adaptations for increased accessibility. We have presented a controlled lab experiment to validate their effectiveness and acceptance in different cost-benefit conditions. The systematic variation of cost-benefit conditions allowed us to investigate the manifold interactions between the utility of an adaptation and possible mechanisms to put the user in control of system-initiated adaptations.

Our study revealed strong support for two of the proposed adaptation patterns: "Automatic adaptation with implicit confirmation" (AI) and "Explicit confirmation before adaptation" (EB). On the basis of their specific advantages in different costbenefit situations, we conclude to use both patterns in our MyUI adaptation framework. And we advise other designers to use corresponding confirmation strategies in their adaptive systems. AI is best for adaptations with great advantages for the user. It provides an unobtrusive notification of the current adaptation and offers the opportunity to undo the adaptation. In situations where high costs for the user are expected, EB should be used. This pattern has proven to reduce the subjective costs of an adaptation and offers very clear control mechanisms by requesting an explicit user confirmation before the adaptation is executed.

Fig. 1 .

 1 Fig. 1. Gloves, glasses and nonsense font (Wingdings) to systematically vary the benefits of user interface adaptations

Fig. 2 .

 2 Fig. 2. Cost-benefit conditions for the adaptation of increasing the font size during the read email task (example)

Fig. 3 .

 3 Fig. 3. Numbers of positive user assessments of transparency, controllability, comfort-of-use and acceptability for the different adaptation dialogue patterns

Table 1 .

 1 Preferred adaptation dialogue patterns (absolute frequencies)

		baseline	AI	EB	EA
	Preference per test case (n=48) *	3	17	17	11
	General preference (n=12)	1	5	5	1

Table 2 .

 2 Adaptation dialogue patterns and their capability to control the costs of adaptations. The table shows absolute frequencies of cases where participants report low subjective costs while being in a high-cost test condition ("Cost subj. < Cost int. ").

		baseline	AI	EB	EA
	All test cases (n=96)
	Cost subj. < Cost int.	6	7	8	8
	Cost subj. ≥ Cost int.	18	17	16	16
	After review 1 (n=48) *
	Cost subj. < Cost int.	1	1	5	6
	Cost subj. ≥ Cost int.	11	11	7	6

1 "After review": the users are aware of the different patterns

Table 3 .

 3 Absolute frequencies of user preferences for the different adaptation dialogue patterns under the experimental conditions of low vs. high intended costs and benefits

		baseline	AI	EB	EA
	Intended costs
	Low (n=24)	2	7	8	7
	High (n=24)	1	10	9	4
	Intended benefits *
	Low (n=24)	1	5	13	5
	High (n=24)	2	12	4	6

Acknowledgments. The research leading to these results has received funding from the European Union's Seventh Framework Programme under grant FP7-ICT-248606. The authors acknowledge the help of other partners in the MyUI consortium in this work.


Nuzhah Gooda Sahib
email: nuzhah@eecs.qmul.ac.uk

Tony Stockman
email: tony.stockman@eecs.qmul.ac.uk

Anastasios Tombros

Oussama Metatla
email: oussama@eecs.qmul.ac.uk

Participatory Design with Blind Users: A Scenario-based Approach

Keywords: scenario, participatory design, visually impaired users

Through out the design process, designers have to consider the needs of potential users. This is particularly important, but rather harder, when the designers interact with the artefact to-be-designed using different senses or devices than the users, for example, when sighted designers are designing an artefact for use by blind users. In such cases, designers have to ensure that the methods used to engage users in the design process and to communicate design ideas are accessible. In this paper, we describe a participatory approach with blind users based on the use of a scenario and the use of dialogue-simulated interaction during the development of a search interface. We achieved user engagement in two ways: firstly, we involved a blind user with knowledge of assistive technologies in the design team and secondly, we used a scenario as the basis of a dialogue between the designers and blind users to simulate interaction with the proposed search interface. Through this approach, we were able to verify requirements for the proposed search interface and blind searchers were able to provide formative feedback, to critique design plans and to propose new design ideas based on their experience and expertise with assistive technologies. In this paper, we describe the proposed scenario-based approach and examine the types of feedback gathered from its evaluation with blind users. We also critically reflect on the benefits and limitations of the approach, and discuss practical considerations in its application.

Introduction

When designing accessible interfaces, it is crucial for the designers to make sure that their understanding of the problem is aligned to the users' experience of their interactions with the interface. When designers interact with systems using different senses, devices and interface widgets than to the target population, it can be difficult for them to exclusively depend on their expertise to correctly imagine the needs of the users and to conceptualise the users' interactions with the system. Thus, designers have to be particularly sensitive as to how the users perceive technology [START_REF] Newell | The use of theatre in requirements gathering and usability studies[END_REF].

As a result, design paradigms such as "Inclusive Design" [START_REF] Clarkson | Inclusive Design: Design for the Whole Population[END_REF][3], "Design for All" [START_REF] Newell | User sensitive inclusive design -in search of a new paradigm[END_REF] and "User Sensitive Inclusive Design" [START_REF] Newell | User-sensitive inclusive design[END_REF] have been proposed to encourage designers to include non-standard populations such as older and disabled users in the design process. These approaches aim to give an effective voice to users in the design process and enable designers to develop real empathy towards users to ensure they communicate design ideas in an accessible form.

In this paper, we propose the use of scenarios for participatory design with blind users. We use a scenario, expressed as a textual narrative, as a basis for dialogue between designers and users in the design of a search interface. There are two levels to our approach to participatory design: firstly, we include a blind user with knowledge of assistive technologies as a full member of the design team and secondly we use the scenario and a dialogue-based interaction to gather formative feedback from 4 blind users.

The details of the finished search interface and its evaluation are reported in [START_REF] Gooda Sahib | Evaluating a search interface for visually impaired users[END_REF] and are not detailed in this paper. Instead, we address the question of how to successfully engage blind users in design, given that the majority of tools used for early stage prototyping by developers, such as wireframes and paper prototypes, contain barriers to participation by blind users. We focus on describing the steps in creating the scenario and its evaluation as a means of engaging blind users in the design process. Therefore, the contributions of this paper are three-fold:

(i) We propose a participatory approach based on a textual narrative scenario and a dialogue-based interaction to engage blind users in the design process. (ii) We evaluate our approach with blind users and describe the types of feedback that we gathered. (iii) We reflect on our approach outlining its benefits, challenges and the practical experiences that we gained from applying it so that the approach can be reused or further developed.

The rest of this paper is structured as follows: in Section 2, we describe the use of scenarios in user-centred design and discuss approaches for engaging blind users in the design process. In Section 3, we outline the steps involved in developing the scenario-based approach and we discuss the user evaluation of the proposed approach in Section 4. We reflect on the approach in Section 5, outlining the benefits, challenges and practical experiences.

Related Work

To the best of our knowledge, in the context of interface design, there is no reported research on the use of scenarios for participatory design with blind users. Thus, in this section, we discuss how scenarios have been used in usability engineering and we describe approaches to participatory design with blind users.

Using Scenarios in Design

The use of scenarios in the early stages of the design cycle involves designers using a description of people (actors) and their activities (tasks) to help potential users to envision an interface that will be developed in the future [7, p. 46]. Scenarios consist of a plot, including a sequence of actions and events, which help to emphasise and explore the goals that a user might adopt and pursue. Scenarios enable rapid communication among different stakeholders and thus, scenario-based design approaches are iterative and lightweight for envisioning future use possibilities [START_REF] Rosson | Scenario-Based design[END_REF]. As a result, designers can work through ideas rapidly, obtaining feedback and refining their ideas to make quick progress. Scenarios focus the design efforts on use, that is, what people will use the interface for and how they will use it [START_REF] Jarke | Scenario management: An interdisciplinary approach[END_REF]. This compels designers to maintain a consideration for people and their needs, as opposed to focusing only on the technology.

Apart from their use in framing the design rationale, scenarios have been used in HCI for other purposes, namely, for planning and evaluating test tasks and to specify usability goals. In [START_REF] Bodker | Scenarios in User-Centred design -setting the stage for reflection and action[END_REF], Bodker highlighted how scenarios can be used at different times with different purposes and described three ways of using scenarios in usability work, namely to generate ideas during field studies, as a starting point in design workshops and for usability testing of prototypes. Scenarios also have a natural and inherent ability to support participatory design as they allow users to identify themselves as the actors in the scenario and to reflect on their own ideas and their implications in the context of design. In this way, scenario-based approaches provide a common language for discussions among users and designers [START_REF] Jarke | Scenario management: An interdisciplinary approach[END_REF]. Newell and McGregor [11] suggested a story-telling approach with older and disabled users to gather information and data about accessibility issues by using scenarios in the narrative form. In this respect, [START_REF] Marquis-Faulkes | Scenario-based drama as a tool for investigating user requirements with application to home monitoring for elderly people[END_REF] used scenario-based drama to elicit user requirements in the design of a fall detector for elderly people. Four scenarios were developed which were performed by a theatre group and filmed. These videos were then used to engage elderly people in the design process by provoking discussions about the use of the system. Other examples of the use of scenarios to engage users in the design process are: [START_REF] Bhatia | VTAssist: a location-based feedback notification system for the disabled[END_REF] used scenarios to understand user requirements in the design of a location-based feedback notification system for users with mobility impairments and in the design of digital technologies for older users, [START_REF] Lindsay | Engaging older people using participatory design[END_REF] used video prompts of a scenario about the problem domain for participatory design with users who were in the 65+ age group.

Participatory Design with Blind Users

Participatory design with blind users can be challenging as designers have to ensure their methods of communicating design ideas with users are appropriate and effective to gather useful feedback. In [START_REF] Okamoto | Possibility of participatory design[END_REF], Okamoto reported about a workshop where scenarios were used by visually impaired and sighted students to discuss products being designed to enhance day-to-day activities for visually impaired users. However, no details of the implementation of the scenario-based method were given, so it is diffi-cult to understand how scenarios were used in that context. In another setting, [START_REF] Metatla | Designing for collaborative crossmodal interaction[END_REF] conducted a workshop including round table discussions and demonstrations of early prototypes to engage visually impaired users in the design of a system to represent diagrams in sound.

Prototyping is also a common way of brainstorming design ideas with users, but for obvious reasons, visual prototyping techniques are not appropriate for blind users and therefore, alternatives have been proposed: [START_REF] Brewster | Guidelines for haptic Lo-Fi prototyping[END_REF] describes haptic paper prototypes (using cardboard mockups) while a tactile paper prototyping approach (with Braille and tactile graphics mockups) was discussed in [START_REF] Miao | Tactile paper prototyping with blind subjects[END_REF]. Also, [START_REF] Tanhua-Piiroinen | Tangible models in prototyping and testing of haptic interfaces with visually impaired children[END_REF] proposed 2 types of haptic mock-ups for visually impaired children consisting of cardboard models and Braille-labelled plastic artefacts.

However, these techniques are time consuming to set up and changes are not easy to make in response to feedback. Both methods proposed in [START_REF] Miao | Tactile paper prototyping with blind subjects[END_REF] and [START_REF] Tanhua-Piiroinen | Tangible models in prototyping and testing of haptic interfaces with visually impaired children[END_REF] exclude the significant proportion of the blind population who are not Braille readers and are also only suitable to prototype haptic interaction as opposed to speech-based screen reader interaction. Also, the cardboard and plastic abstract models such as those used in [START_REF] Tanhua-Piiroinen | Tangible models in prototyping and testing of haptic interfaces with visually impaired children[END_REF] have a possible drawback of not allowing users to fully conceptualise the application as a whole, since users only interact with individual artefacts at a time.

In the following section, we describe our approach to engaging blind users in the design of a search interface. As well as including a blind user in the development team, we used a textual narrative scenario in a participatory design setting as a basis for dialogue to simulate interaction, in order to discuss design ideas with potential users.

Using Scenarios for Participatory Design with Blind Users

In this section, we describe the development of the proposed scenario-based approach. In Section 3.1, we explain the rationale for our overall approach to participatory design and in Section 3.2, we outline the steps included in developing the proposed scenario-based approach to verify requirements with blind users in the design of a search interface.

Rationale for the Participatory Design Approach Taken

To access the Web, blind users depend on a screen reader, a software application that by default reads web pages linearly from left to right, top to bottom, rendering the content in computer synthesised speech or Braille. Blind users also use the keyboard to navigate web pages and position the focus of the screen reader to read parts of the page of interest. Typical commands supported by screen readers include web page navigation forward/backward by headings (at different levels), forms, frames, edit fields, buttons and links. The linear rendition of text by screen readers plus the fact that they do not represent the spatial layout of web pages, such as columnised format, means that the mental models of blind users can vary significantly from those of sighted users [START_REF] Stockman | The influence of Screen-Readers on web cognition[END_REF].

There is a parallel to be drawn here between web navigation and navigation of real world spaces. Given due consideration, it is unlikely that when giving directions to a pedestrian, the way in which one would describe those directions would be the same for a sighted pedestrian as for a blind pedestrian. Instructions to the sighted pedestrian are likely to exploit visual cues, to be given at a granularity level appropriate to someone who can take in their surroundings at a glance. On the other hand, directions to a blind pedestrian, if they are to be useful, should be in terms of landmarks that are detectable by them, and at a level of granularity related to the way in which they interact with their surroundings, given whichever mobility aid they might employ, be it a dog or a white cane etc. Similarly, within human-computer interaction, in order to be useful, the way in which interactions are articulated need to take into account the senses and tools at the disposal of the user, as well as the level of granularity at which they interact with the system.

Based on this need to embed an understanding of how end users interact with the system at a deep level and the fact that other members of the development team can not easily share that experience (using a screen reader with a covered screen is not a realistic surrogate for a blind user with thousands of hours of screen reader experience [START_REF] Petrie | The use of non-speech sounds in non-visual interfaces to the MS-Windows GUI for blind computer users[END_REF]), it was decided that participatory design should be addressed at two levels. Firstly, we included a blind user with knowledge of assistive technology as a full member of the design team. This provided the development team with immediate feedback in discussions about the development of appropriate inter-face artefacts, for example, properly labelled controls, the types of interactions supported by screen readers (the use of screen reader commands for web page navigation) and the appropriate vocabulary with which to describe interactions to blind users, for example, keystrokes rather than mouse clicks.

This understanding of how screen reader interaction works led to the development of a scenario and a dialogue about it being pitched at an appropriate level to make sense to a screen reader user. For example, the interface comprised several different components such as a search box, to which the user would frequently want to navigate. In this case, knowledge of screen reader interaction suggested that the appropriate way for this to be achieved should be through a keyboard shortcut and that an appropriate means of confirming that the action has been executed could be through playing a non-speech sound.

The second level at which participatory design was achieved was through the recruitment of 4 blind participants who took part in prototyping sessions to provide formative feedback to the design team. In these sessions, the overall scenario was used as the basis of dialogue about how users would interact with the system using a screen reader and the usefulness and usability of proposed interface features.

3.2

Creating the Scenario-based Approach

During the requirements verification stage of the design process, the requirements of a system are analysed and validated to ensure that the designers and the users share the same understanding of the problems that were identified during the requirements gathering stage. For participatory design approaches, at this stage, designers communicate early design ideas to users to gather feedback [START_REF] Pekkola | Towards formalised end-user participation in information systems development process[END_REF].

In [START_REF] Gooda Sahib | A Comparative Analysis of the Information Seeking Behaviour of Visually Impaired and Sighted Searchers[END_REF], we identified user requirements for a search interface for blind users through an observational study and in this paper, we verify requirements by using a scenario expressed as a textual narrative which then formed the basis of dialogue between the designers and the users. Basing this dialogue on a narrative scenario evoked a form of role play which worked well because the human mind is adept at overloading meaning in narrative structures [7, p. 54] which are meant to stimulate the imagination [START_REF] Jarke | Scenario management: An interdisciplinary approach[END_REF] and to provoke new ideas [START_REF] Bodker | Scenarios in User-Centred design -setting the stage for reflection and action[END_REF]. Therefore, they are well suited for use in participatory approaches to engage users early in design.

Our approach is a hybrid one involving a combination of participatory design [START_REF] Muller | Participatory design: The third space in HCI[END_REF] and the use of a detailed scenario to discuss ideas with target users [START_REF] Carroll | Making use: scenario-based design of human-computer interactions[END_REF]. The participation of a blind user as a member of the design team was invaluable when developing the scenario and its associated textual narrative as it helped us to conceptualise how potential users will interact with the system, given their use of screen readers. It also allowed us to establish the level of detail at which the scenario should be discussed with end-users. In Figure 1, we provide a broad overview of the framework we followed to implement the scenario-based approach and in the following we describe how we implemented each step of the framework in the design of a search interface for blind users. For each step, we also highlight the contributions of the blind member of the design team. Step 1: Identify set of interface features. From our observations with blind users [START_REF] Gooda Sahib | A Comparative Analysis of the Information Seeking Behaviour of Visually Impaired and Sighted Searchers[END_REF], we identified a set of search interface features that could support blind searchers during information seeking on the Web. These interface components were chosen to address the difficulties observed in [START_REF] Gooda Sahib | A Comparative Analysis of the Information Seeking Behaviour of Visually Impaired and Sighted Searchers[END_REF] and were influenced by the design team's intuitions and knowledge of search user interface components. During this process, the blind team member contributed significantly from his knowledge and experience of using both graphical interfaces (via screen readers) and self-voicing auditory interfaces. This, to some extent, allowed the sighted designers to conceptualise the mental model they had to follow when designing interface components.

Step 2: Create detailed description of features. To communicate ideas for the search interface, we created detailed descriptions for all interface features. As we were using a textual narrative scenario, we had to ensure that our ideas were being conveyed correctly to the users and therefore, we focussed significantly on describing each interface component. For example, we proposed an integrated note-taking feature within the interface as described in the following:

In this step, the contributions of the blind team member were significant in discussing the functionality of suggested support features as well as how searchers would interact with them. Through such discussions, we could ensure that interaction components were appropriate, and that the correct vocabulary was being used to describe interface components.

Step 3: Refine description iteratively. To ensure that the users shared the same understanding of the proposed features as the designers, we iteratively refined the description of the features through several informal conversations with the blind team member. For example, one idea was that a context menu might be useful to provide access to a set of options that become available when exploring individual search results. The idea of doing this through a context menu was contributed by the blind team member, who highlighted that context menus were familiar interaction artefacts to most screen reader users.

The blind team member further contributed that the way to initiate the interaction with end-users about the context menu should be by telling them to use a key combination (Shift+F10), rather than right clicking, as the keystroke is the usual way a blind user will initiate the interaction compared to the right mouse click familiar to sighted users. Therefore the blind team member gave us both an appropriate interaction artefact, and the most fitting means of describing the interaction to end-users. The options to be made available through the context menu were then identified and the best ways of implementing and describing the interactions to end-users were then refined through discussions between the sighted designers and the blind team member. The following was the final description used for the context menu:

Searchers can create a note and the system asks them where they would like to save this note and to give it a name. The note is divided in two parts: The first part of the note is editable by the searcher, that is, they can type ideas, copy and paste things from web pages etc. The second part cannot be edited and is used to save search results automatically by the system.

To enhance our textual description of some search support features, we referred to examples from other popular interfaces such as Google Search (results presentation with title, short description and web address) and Windows (context menu) that the users would be familiar with. These familiar points of reference helped the blind users to better envision the proposed search interface.

Step 4: Construct scenario around interface features. Once we finalised which search support features to include on the interface, we created an overall scenario, like a story, with a specific setting whereby the user was using a search interface for the first time after hearing about it from a friend. As we were also evaluating a new history mechanism and interface features to support searchers in resuming search tasks [START_REF] Morris | SearchBar: a search-centric web history for task resumption and information re-finding[END_REF], the scenario included a stage where the user had to leave the task midway to attend an important appointment. When constructing the overall story, it was also essential to ensure the story included all suggested interface features in the correct order and in a reasonable sequence. For example, we would not describe a feature for managing search results before the users were asked to submit their first query.

Step 5: Dialogue-simulated interaction with the scenario. After the overall scenario was constructed, we used it as the basis of a formative evaluation with potential users (described in Section 4). The evaluator conducted a dialogue with each potential user to simulate the interaction with the interface components proposed in the scenario. At each step of the interaction, the evaluator would describe the interface feature to the user and explain how to interact with it. Then, the evaluator would ask the user for feedback on the feature and they would discuss the alternative interaction paths resulting from multiple design ideas.

Evaluating the use of Scenarios for Requirements Verification

We evaluated the approach through a dialogue with potential users using the scenario to simulate interaction between the users and the yet-to-be constructed interface. Our goals for the evaluation were two-fold. Firstly, we wanted to verify the requirements for a new information-rich search interface for blind users, to communicate and discuss design ideas with potential users early in the design process. Secondly, our aim was to evaluate the use of scenarios in a participatory design setting for engaging blind users in the design process. Hence, in Section 5, we discuss the benefits, challenges and practical experiences of using the approach proposed in this paper.

You are aware that this new interface has a menu associated with each search result so that you can open, save, email and copy results. You hit the menu key and you find the following options in this particular order (Save Result, Copy, Email, Open). This is rather like the context menu you have in Windows that you bring up using Shift+F10.

4.1

Participants

We recruited 4 blind users through word of mouth and via online email lists. The participants were experienced searchers who rated their proficiency with assistive technologies from intermediate to advanced. Three of the participants were educated to a postgraduate level while one had professional qualifications in IT. In Table 1, we provide additional demographic information about these participants.

Procedure

For each evaluation session, we used a standard script of the final scenario (described in step 4 of Section 3.2) to ensure that the users and ourselves shared the same understanding of the requirements for a new search interface. To begin with, the evaluator asked the user to think of a search task to complete. We left the choice of task open to elicit greater participation and user engagement with the scenario. The choice of search task did not affect the use of the script as it was built so that its primary focus was on interaction with individual interface components and thus could be adapted to any search task.

During the session, the evaluator who was the one in charge of the script, started the conversation with the participant by conducting a walk-through of the scenario in line with the script (The beginning of the dialogue-simulated interaction between the user and the evaluator is illustrated in Table 2). At each step, the evaluator provided the user with complete descriptions of the search interface feature and the user was prompted for feedback. The evaluator and the user also discussed how each interaction would work, including alternative interaction paths in case of multiple design ideas as shown in Table 3.

Table 2. Excerpt on query specification.

Evaluator: Your friend has told you about a new search system and you would like to try it out for yourself to see how good it really is. Think of something you would like to search on this new system.

Once you have chosen your search task, you type the address of this new search inter-face in your web browser and you reach the page with the cursor in the search edit box.

"What do you type as a query?"

User: digital rights accessibility

Evaluator: You type this query and hit enter. If you misspell a word in your query, the system will specify which term you misspelt and allow you to submit a corrected version of your query.

User Feedback

In this section, we describe the feedback gathered from 4 blind users during the evaluation of the scenario-based approach. The dialogue-simulated interaction between the designers and the users allowed us to gather feedback from users through their comments and critiques of suggested design plans. Additionally, we also use our own observations to categorise the user feedback as presented in the following:

Verifying requirements. The scenario-based approach allowed us to verify requirements for a search interface for complex search tasks. We identified the requirements for a search interface during an observational study with 15 visually impaired searchers [START_REF] Gooda Sahib | A Comparative Analysis of the Information Seeking Behaviour of Visually Impaired and Sighted Searchers[END_REF]. Through the proposed approach, we were able to ensure that the design team and the target users shared the same understanding of the difficulties faced by blind users when using current search interfaces.

In this respect, we were able to, for example, ascertain that spelling suggestions were a source of difficulties for searchers as the way misspelt words are rendered on current interfaces is not intuitive for screen reader users. One of the users said: "we hardly notice which term is misspelt. It would be good if the system clearly said which term is wrongly spelt". This is because when spelling suggestions are presented to the user, the way in which the screen reader pronounces the suggestion for the correctly spelt word is often not detectably different from the pronunciation of the original misspelling, and so it is not clear what error is being corrected. In this case, the blind searcher can navigate to the suggestion and cursor character by character through it to find the difference, but this process loses all the immediacy of the visual representation. This was a difficulty that we had observed in [START_REF] Gooda Sahib | A Comparative Analysis of the Information Seeking Behaviour of Visually Impaired and Sighted Searchers[END_REF].

Likewise, we were able to verify user requirements for a new history mechanism. In our scenario, we proposed a search history mechanism that would keep track of the queries submitted and the search results visited by the searcher. Participants in the evaluation commented on the need for such a history feature saying "I do not like the history in IE, this is more powerful than history. It allows you to call it up and instantly be back to where you were, in the same context" and "It is nice to pick up from where we left because sometimes we use keywords which are useful and then forget the right combination".

Identifying issues with current design ideas. By discussing design plans with potential users early in the design process, we were also able to identify issues with proposed design ideas from the users' perspective. Such discussions proved to be beneficial; for example, one of the design ideas for search results presentation included limiting the display of individual results to only one line per result on the search results page. Our reasoning for this idea was that it would reduce the amount of text that screen reader users would have to go through. However, we found that participants in the evaluation did not welcome this idea, as they would rather have some context about the search results retrieved by the search engine. They felt that if there was only one line per result, there would not be enough context to decide about the relevance of a specific result among those that the search engine had retrieved.

During the dialogue-simulated interaction, the evaluator also had the opportunity to probe users on factors like keyboard navigation, which plays a central part in the user experience of blind searchers. Keyboard navigation is significantly different from visual navigation and hence, the design team have to ensure that all proposed interface features were intuitive to access via the keyboard.

In describing how they would interact with the interface, participants would often refer to how they would use the screen reader to access the proposed features. About the grouped approach for results presentation, one participant said "Along the lines of how VoiceOver works, this grouping on the page would be good" and another questioned how they would navigate back to a previous page "Would I need to use the screen reader key for this or would there be a special key combination?".

New design ideas proposed by users. Using the scenario-based approach allowed us to engage users in the development process via their interaction with a yet-to-be constructed search interface. Through this process, users came up with ideas of their own to enhance the design of some of the features that were being proposed. For example, in the scenario, we described a note-taking feature, which could be used by searchers to automatically save search results or to make notes of their own.

The initial idea was to allow users to then download or email the note in a text format. However, one of the participants highlighted that the benefits of having an integrated note facility could be enhanced by structuring the note and by including HTML tags to allow users to easily get back to any previously accessed web pages. About the same note-taking feature, another participant augmented our basic definition of the feature with his own design ideas, suggesting, "I can see where you are going with this, it could be in two panes, your browser and your search notes". The user was in fact proposing that there should be two separate areas on the interface, one for regular browser-related activities such as submitting queries and viewing web pages and the other area should be dedicated for note-taking and other search management activities. When users suggested such design ideas of their own, we dis-cussed them with the design team including the blind co-designer to ensure that such an approach would be feasible and would enhance the users' experience.

Table 3. Excerpt on alternative search results presentation.

Evaluator: There are alternative ways of presenting the search results retrieved:

1. Standard approach: Results are presented in a list with each result described using a title, a short summary and a web address. Each of these items is on a separate single line. 2. Simplified standard approach: Results are presented in a list, but each result is described in one line, with a title and a short summary. 3. New approach: Similar results are grouped together and you are presented with an overview of each group of search results. For example, results that deal with similar topics will be grouped together. If you are doing a travel task, web pages describing things to do at your destination will be grouped together and another group of pages could be about possible places to stay. If you would like to explore one of these groups, you can select the group and it will open in a different window and will contain all search results in that group described with title and a short summary. You can always return to the first window to browse through other result groups.

"What are your thoughts on these results presentation alternatives? Which one would you prefer and why?"

Discussion

Engaging non-standard populations such as the elderly and disabled users in the design process is challenging as traditional user methodologies are not always effective at capturing the real user requirements. Therefore designers often have to explore different methods or adapt existing ones to ensure that such users can be successfully included in the design process [4][14].

In this paper, we described an approach, which included 2 levels of participatory design: we included a blind user in the design team and also carried out prototyping sessions with 4 blind users. Involving a blind team member who can combine a good knowledge of assistive technologies with an end-user perspective enabled us to create a scenario that was better matched to the vocabulary and interactions familiar to blind users. Thus, we could successfully engage blind users to solicit their feedback in the design of the search interface.

Search interfaces are highly interactive and to progress in their search task, searchers are required to perform activities such as formulate queries and view search results etc. The scenario-based approach described in this paper allowed us, to some extent, to simulate this interaction through a dialogue between the user and the designer. During the dialogue-based interaction, users were involved in the scenario and were constantly informed about their evolving interaction, for example, how search results are being handled and the alternative paths available to them. This approach to interaction elicited a high level of participation and engagement from the users, as evidenced by the feedback received. Therefore, such use of dialogue was beneficial as a model of engagement [START_REF] Wright | Experience-centered Design[END_REF] and a model for effective communication and collaboration [START_REF] Anderson | Dialogue: Theorizing Difference in Communication Studies[END_REF] between the designers and the potential users.

Overall, the findings gathered from the dialogue-simulated interaction showed that users had no problems in 'imagining' the interface proposed in the scenario [START_REF] Carroll | Making use: scenario-based design of human-computer interactions[END_REF]. The narrative was successful in evoking the search experience in users and therefore, they were able to discuss the proposed features for inclusion in the interface in the context of their use within the scenario, and to discuss alternative interaction sequences where they arose. The fact that users were able to go beyond the described interface features to question how they would interact on a relatively low level (screen reader keystroke level) is evidence that they were able to successfully form a mental model of the search interface that was yet-to-be constructed.

In addition, involving blind users at such an early stage allowed designers to identify limitations with their own design ideas. Participants would often question the practicality of the proposed interface features, requiring detailed explanations of how these interface components would be accessed in a realistically usable way with screen readers. Identifying these limitations at that stage ensured that no further development effort was put into interface features that would not meet the needs of the users, or would raise difficult usability issues.

The benefit of an inclusive approach, such as the one proposed in this paper, is that it enables users, especially those with disabilities, to become involved in the process of design and formative evaluation. This involvement in the development process encourages users to speak about their experiences with search interfaces and to contribute to design ideas and hence, the user truly becomes the centre of the design process. User-generated ideas during the scenario walk-through resulted in valuable contributions to our design plans. This is so because the participants in our study were experts at navigating the Web through screen readers and given their experience, they had better insights into how the overall search interface and the individual components would be perceived.

Reflections on the Use of Scenarios for Participatory Design with Blind Users

In this section, we reflect on the scenario-based approach and its evaluation with potential users. We discuss the benefits, challenges and practical experiences of using scenarios to engage blind users in the design process.

Benefits

Scenarios are flexible and adaptable and thus they can be customised according to the needs and abilities of the user group, for example, as a scenario-based drama for the elderly [START_REF] Marquis-Faulkes | Scenario-based drama as a tool for investigating user requirements with application to home monitoring for elderly people[END_REF]. For our project, we created a textual narrative scenario for a dialoguebased interaction with blind users. The value of the scenario was that it allowed blind users to envision the proposed interface and form a mental model of how they would interact with it. This was important to correctly verify user requirements with blind users and also to rapidly communicate design ideas.

In addition, scenarios are adaptable in the level of detail that they convey to the user group, which can assist in enabling them to envision the proposed artefacts. For our approach, given our focus on requirements verification, we provided detailed descriptions for the proposed interface features and less detail about the interaction or the way certain tasks could be completed when using the search interface. For example, when describing a new search history mechanism, we fully described the items such as the queries and visited results that would be recorded as history, but we did not explicitly tell users how they would navigate the trail at a keystroke level. Instead, during the sessions, the users themselves wondered and discussed how they would interact with this history mechanism for different types of tasks.

In this way, we were able to achieve the comparable 'unfinished look' of handwritten mock-ups that Snyder [START_REF] Snyder | Paper prototyping: the fast and easy way to design and refine user interfaces[END_REF] claims encourages creativity during low-fidelity paper prototyping. However, depending on the users' needs and the stage of the design process, a scenario-based approach could be used for more high-fidelity prototyping to evaluate how users would interact with the proposed artefacts. Our discussions with participants regarding how some interface features could be accessed through screen readers show that the use of scenarios is likely to be effective for such high fidelity prototyping.

In the absence of visual aids to communicate design ideas, sighted designers are likely to describe graphical user interfaces in a way that makes references to visual aspects of the interface, such as layout, structure etc. For the blind user, these descriptions would not be useful and would not convey a helpful representation of the interface features. For this reason, the involvement of a blind user was crucial to ensure that we used the right vocabulary and context to describe interactions at an appropriate level from the user's perspective.

Scenarios, especially when expressed as narratives, have an inherent ability to support participatory design [START_REF] Jarke | Scenario management: An interdisciplinary approach[END_REF] [START_REF] Luck | Dialogue in participatory design[END_REF] and thus complemented the level of participatory design reported in this paper. In such settings, scenarios furthered the communication between the users and the designers to enable successful collaboration [START_REF] Anderson | Dialogue: Theorizing Difference in Communication Studies[END_REF]. We expressed the scenario in the form of a textual narrative (which was then used as the basis of a conversation between designers and users) and this enabled blind users to comment on the proposed design ideas in the context of screen reader access, as well as to suggest their own ideas for new or modified interface components.

Challenges and Practical Experiences

In the absence of visual aids, the designers in this approach relied on the textual descriptions of the interface features to communicate design ideas to the users. Therefore, the detailed descriptions played a significant role in shaping the mental model that users created of the interface. Using a standard script for the scenario ensured that variations in the way the interface was conceptualised were limited.

Our approach focussed entirely on the functionality of interface components and the way to interact with them. No efforts were directed towards conveying spatial information, which despite not necessarily being of primary importance to blind users, plays a role in how screen reader users perceive an interface, and very importantly, their collaborative use of the interface with sighted peers [START_REF] Stockman | The influence of Screen-Readers on web cognition[END_REF]. As an extension to this work, it will be interesting to examine the benefits and drawbacks of incorporating screen reader technology within the prototyping process, rather than the purely conversation-based approach taken here. It is unclear whether the incorporation of screen reader technology will enhance the realism of the interactions, and/or whether it may detract from the free flow of the dialogue about the interactions and their possible alternatives by overburdening the audio channel [START_REF] Chandrashekar | Using think aloud protocol with blind users[END_REF].

The approach proposed in this paper was a first attempt at using scenarios to engage blind users in the design process and hence, we identified some important points to consider for any future implementation or extension of this approach. Firstly, we expressed the scenario in a textual medium, with a dialogue-simulated interaction between the user and the designer. This audio-based approach works well with blind users, but as is common with audio interfaces, there is a lack of persistence. Therefore, any artefacts that are part of the scenario should be described in significant detail to ensure that users can conceptualise and "picture" the proposed design. Visual aids such as paper mock ups convey significant contextual information even in their most early versions and any attempt at replicating these types of approaches for blind users should be constructed using low-level details in the textual descriptions. Detailed descriptions can also be complemented with references to similar existing artefacts to convey as much contextual information as possible.

From the user evaluation, we concluded that scenarios, especially those expressed as a narrative, should be highly interactive to include the user as much as possible. Given that scenarios are stories about people and their activities, it is essential for users to feel part of the scenario to maximise their ability to envision the proposed interface. In the scenario, we regularly prompted users for feedback by asking them to think of a search task, by asking them for their query terms and by allowing them to choose the next step of their interaction etc. When scenarios are textual narratives and interaction with the user is dialogue-based, the designer will be speaking for relatively long periods to describe different parts of the interface. Therefore, to replicate an interactive search experience, users should be active 'actors' in the scenario activities to further user engagement.

Involving a blind person on the design team helped in many ways, but it is important to be aware of the dangers of over-relying on one person as a representative of a population. For example, the blind co-designer in our team was congenitally blind and had a lot of experience using JAWS with Windows and Internet Explorer to perform searches using Google, but only a passing knowledge of other screen readers, browsers and search engine combinations. Therefore, it is important to try to ensure relevant diversity [START_REF] Lindsay | Engaging older people using participatory design[END_REF], that is, users involved in the prototyping process, together with members of the design team, should provide a wide coverage of the range of tools and assistive technologies that might be used with the system being designed. It is also important to include users with less experience, as they will also be representative of members of the target population.

Conclusion

In this paper, we proposed a participatory approach based on a textual narrative scenario, tailored to the abilities of blind users to engage them in the design process. We evaluated the proposed approach with blind users and described the types of feedback we gathered in a participatory design setting through the use of a scenario as a basis for dialogue. The dialogue-simulated interaction between designers and users was effective in evoking the search experience in users and thus they could envision the yet-to-be constructed interface. We achieved two levels of participatory design, namely by including a blind user in the design team and by carrying out prototyping sessions with 4 blind users. The contributions of the blind team member were invaluable to ensure that, in constructing the scenario, we used the right vocabulary and context to describe interactions at an appropriate level for screen reader users. In this paper, we also reflected on the benefits and challenges of our proposed approach and the practical experiences we gained in applying it so that it can be reused or further developed.

We believe that the proposed approach opens an interesting discussion on the ways to adapt current tools and techniques in user-centred design when designing for nonstandard populations such as the elderly or users with disabilities. In this respect, future work could also focus on the comparison between different techniques to investigate the best ways of engaging users with disabilities in design depending on the stage of the design process and the type of feedback required.

Fig. 1 .

 1 Fig. 1. Framework for the scenario-based approach.

Table 1 .

 1 Demographics for all participants.

	Age	37 years
	Gender	M (3) F (1)
	Search Experience	12 years
	Screen Reader	JAWS (3) VoiceOver (1)
	Frequency of Computer Use	Daily (3) Weekly (1)
	Use of Online Search Engine	Daily (3) Weekly (1)