Prototyping for Design and Evaluation

• What is Prototyping
• Introduction to Prototyping Techniques
• Low-fidelity prototyping
• Sketches
• Storyboard
• PICTIVE
• Steps of building a low-fidelity prototype
• Comparison of low-fidelity prototyping techniques
• Medium-fidelity prototyping
• Computer-based simulation
• Wizard of Oz
• Slide shows and video prototyping
• Comparison of medium-fidelity prototyping techniques
• When, Where and How to Use the Prototyping Method
• When to use prototyping techniques
• Comparison of low-fidelity and medium-fidelity prototyping
• Considerations when choosing prototyping techniques
• Integrate prototypes and products
• The Performance of Prototyping on Some Attributes
• Working Examples
• Windows 95 user interface prototyping and usability testing
• Design of network scenario generation tool
• Related Links
• References

Prototypes are experimental and incomplete designs which are cheaply and fast developed.  Prototyping, which is the process of developing prototypes, is an integral part of iterative user-centered design because it enables designers to try out their ideas with users and to gather feedback [1].

The main purpose of prototyping is to involve the users in testing design ideas and get their feedback in the early stage of development, thus to reduce the time and cost. It provides an efficient and effective way to refine and optimize interfaces through discussion, exploration, testing and iterative revision [2]. Early evaluation can be based on faster and cheaper prototypes before the start of a full-scale implementation. The prototypes can be changed many times until a better understanding of the user interface design has been achieved with the joint efforts of both the designers and the users.

Prototyping can be divided into low-fidelity prototyping, medium-fidelity prototyping and high-fidelity prototyping.  In some literature, it is only simply classified as low-fidelity prototyping (also called Lo-Fi) and high-fidelity prototyping (also called Hi-Fi), where low-fidelity prototyping is mainly about paper-based mock-up, and high-fidelity is mainly about computer-based simulation. The determining factor in prototype fidelity is the degree to which the prototype accurately represents the appearance and interaction of the product, not the degree to which the code and other attributes invisible to the user are accurate. On this web page, we will consider a fully-functioned prototype as a high-fidelity prototype. Other prototypes will be divided into low-fidelity and medium-fidelity prototypes. We will focus on the low-fidelity and medium-fidelity prototyping techniques. Medium-fidelity and high-fidelity prototypings are discussed together on some attributes indicated as medium(high)-fidelity prototyping.

Low-fidelity prototypes are quickly constructed to depict concepts, design alternatives, and screen layouts, rather than to model the user interaction with a system.  Low-fidelity prototypes provide limited or no functionality. They are intended to demonstrate the general look and the feel of the interface, but not the detail how the application operates. They are created to communicate and exchange ideas with the users, but not to serve as a basis for coding and testing. A facilitator who knows the application thoroughly is generally needed to demonstrate the prototype to the users [2].

In contrast, high-fidelity prototypes are fully interactive, simulating much of the functionality in the final product.  Users can operate on the prototype, or even perform some real tasks with it. High-fidelity prototypes are not as quick and easy to create as low-fidelity prototypes, but they faithfully represent the interface to be implemented in the product. Medium-fidelity prototypes partially simulate the system interaction and functionality.

Figure 1 shows the transition of techniques from low-fidelity prototyping to high-fidelity prototyping. However, the fidelity degree of each technique may vary in diffirent practice.  All of the techniques except the fully-functioned high-fidelity prototyping will be discussed on this web page.

 

Figure 1.  Transition of Prototyping Techniques
 
 

Low-fidelity prototyping

Sketches

Sketching techniques, a kind of visual brainstorming, can be useful for exploring all kinds of design ideas. After producing initial sketches the best ideas can be further developed by constructing cardboard representations of the design, which can be evaluated with users. This can then be followed by developing scenarios, software or video prototypes.

Freehand sketches are essential for crystallizing ideas in the early stages of design. Through the act of putting ideas down on paper and inspecting them, designers see new relations and features that suggest ways to refine and revise their ideas. Sketches make apparent to designers not only perceptual features but also inherently non-visual functional relations, allowing them to extract functions from perception in sketches.

The type of mock-up depends on how advanced the idea is. It may be quicker and cheaper to use paper-and-pencil forms at early stages, whereas computer-based prototypes may be important in later stages for exploring and demonstrating interaction and design consistency.

As one can imagine, the sketch technique is as simple as drawing the outward appearance of intended system on paper. However, creativeness is needed. There are some useful training exercises in [1] to help designers get used to visual thinking.  Figure 2 is a sketch of a screen design.

Figure 2.  A Sketched Screen Design
 
Besides paper-and-pencil work, sketches can also be made with the aid of computer software, such as the Paint package in Windows^(R) and SILK [11]. SILK allows designers to quickly sketch an interface electronically and interactively. Figure 3 is a sketch created by SILK.

 
 Figure 3. Sketched Application Interface Created with SILK

Storyboard

Storyboard origins from the film industry, where a series of panels roughly depicts snapshots from an intended film sequence in order to get the idea about the eventual scene.

Storyboard is a graphical depiction of the outward appearance of the intended system without accompanying system functionality. Storyboard provides snapshots of the interface at particular points in the interaction so that the users can determine quickly if the design is heading in the right direction.

Storyboards do not require much in terms of computing power to construct, in fact, they can be mocked up without the aid of computers. The materials needed are office stationery, such as pens or pencils of different colors, Post-It^TM, stickers, and so on. However, modern graphical drawing packages make it possible to create storyboards with the aid of a computer instead of by hand. It is also possible to provide crude but effective animation by automated sequencing through a series of snapshots [3]. For example, SILK [11], as we mentioned above, provides electronical storyboarding function.

Figure 4 illustrates how storyboard is used to represent the system function and sequence .

 

                        Figure 4. An Example of Storyboard

 
 
PICTIVE

PICTIVE stands for Plastic Interface for Collaborative Technology Initiatives through Video Exploration. It was developed at Bell Communications Research (Bellcore) in 1990 within the context of participatory design. The initial experiments of PICTIVE were conducted by Muller and his group in their projects [4]. It is an experimental participatory design technique that is intended to enhance user participation in the design process.

The rationale behind PICTIVE is the methodology of participatory design. PICTIVE insures that users have early exposure to the target implementation technology. The PICTIVE technique provides a fine-grained, dynamic paper and pencil concretization mock-up of what the system will eventually look like and how it will behave. The components are literally made of colored plastic. Their relative durability and inexpensiveness encourage an atmosphere of exploration and invention. The PICTIVE mock-up is intended to be extensively modified in real-time by the users. PICTIVE is less as a means for the evaluation of an already-designed interface, but rather for the creation of the design of an interface.

PICTIVE was begun in reaction to software rapid prototyping environments, in which developers have a disproportionate design impact, but non-computer users are relatively disempowered  by the complexity of current software prototyping environments. The PICTIVE techniques were designed to be used by people who were not necessarily programming professionals, so all participants have equal opportunity to contribute their ideas.
 
The apparatus for PICTIVE includes video camera and a collection of design objects. The design objects fall into two categories. The first category is simple office materials, including pens, high-lighters, papers, Post-It^TM, stickers. labels and paper clips -- all in a range of bright colors. The second category is materials prepared by the developer, such as menu bars, dialogue boxes, special icons for the project and so on.

The procedures of PICTIVE are as follows.
First, both the users and the developers are asked to prepare a "homework". Typically, the users are asked to think about task scenarios, for instance, "what you would like the system to do for you and what steps are required to finish the job". The developers need to construct a preliminary set of system components for the users to manipulate based on prior discussions with the users. Figure 5 shows some of such components made of plastic [4].
Second, during the PICTIVE session, both the users and developers manipulate the design objects on a design surface, where the designs are put together as multiple layers of sticky notes and overlays that can be changed by simple colored pens. Each participant brings her or his expertise. The resulting design is not driven by any single participant, but represents a synthesis of the different participants' different views. The users' work scenarios are explored in detail, leading to a discussion of how the developers' technology can be applied to meet the users' human needs. A coordinator may be needed to keep the group on track.
A video camera is focused on the design objects and to record the voices of the design team as they manipulate those objects. The video record of the design session will serve as a dynamic presentation of the design. Figure 6 illustrates a scene in the PICTIVE session [4].

Figure 5. PICTIVE Plastic "Icons"
 

 

Figure 6. PICTIVE Setting

 
 
PICTIVE is especially suited for prototyping activities carried out as part of a participatory design process since the low-tech nature of the materials make them equally accessible to both the users and the developers. In general, PICTIVE appears to be appropriate in circumstances similar to those of knowledge-based system development, i.e., when there are users available who understand what they need from the product or the project .

More studies on PICTIVE had been conducted and described in [13] . In [14], an experimental object-oriented software prototype named TelePICTIVE was introduced. TelePICTIVE was designed to allow naive as well as expert users to work together in designing GUIs based on the PICTIVE paper participatory design methodology. [15] introduced how to practice design exercises derived from the PICTIVE method.
 

Sample steps of building a low-fidelity prototyping [9]

Assemble a kit

- white, unlined, heavy paper, size 11 by 17 inches is nice
- hundreds of 5-by-8-inch cards as construction material or notes cards
- various of adhesives: tape, glue sticks, Post-It^TM, white correction tape
- various of markers: colored pens and pencils, highlighters
- lots of sticky note pads of various sizes and colors
- acetate sheets to make overhead presentations
- scissors, knives, straightedges, Band-Aids^TM

  Set a deadline

No matter how hard you think about it, you are not going to start getting it right until you put something in front of actual users and start refining your idea by their experience with your design.

Construct models, not illustrations

Make a working model, for example, make the menus dropping down, dialogs popping up, selection highlights and so forth. Photocopier can be useful.

Preparing for a test

Select people who are the users or similar to the users to test your prototype.
Write a set of scenarios, describing the product during use in a typical work situation.
Conduct several dry runs before tests with people outside. Check if there are missing components, confusing representation, etc. Make people in the team familiar with the prototype.

Conducting a test

Team members are assigned different tasks. The greeter welcomes and tries to put users at ease. Facilitator takes the lead, giving user instructions, encouraging them to express their thoughts, making sure everything gets done on time. One team member acts as the "computer". He or she knows the application logic thoroughly, and sustains the illusion that paper prototype behaves similar to a real computer. The rest of the team are observers who take notes.
It is recommended to use a video camera during the test.

Evaluating results

Sort and prioritize the note cards. The team works through the piles of cards and agrees on suggested changes.
 

Table 1 gives a brief comparison of the three low-fidelity prototyping techniques.

 

Techniques	Advantages	Disadvantages
Sketches	*very simple & cheap	*must concentrate on high    level concepts  *hard to envision the    progression
Storyboard	*simple & cheap  *users can evaluate the    direction of the interface is    heading	*can only roughly display the    system interaction  *HCI expertise needed
PICTIVE	*simple & cheap  *fine-grained, dynamic    paper represents the system    interaction  *encourage users' modification	*video needed  *HCI expertise needed

 
                                  Table 1: Comparison of Low-fidelity Prototyping Techniques
 

Medium-fidelity prototyping

Computer-based simulation

Medium-fidelity prototypes simulate or animate some but not all features of the intended system. There are three approaches to limit prototype functionality [6].

Vertical prototyping

Vertical prototyping cuts down on the number of features, so that the result is a narrow system that includes in-depth functionality, buy only for a few selected features.
Vertical prototypes allow users to perform and test some real tasks.
 

Horizontal prototyping

Horizontal prototyping reduces the level of functionality so that the result is a surface layer that includes the entire user interface to a full-featured system without underlying functionality.
Horizontal prototypes allow users to feel the entire interface, even though they can not perform any real tasks.
The main advantages of horizontal prototypes are that they can be implemented fast with the use of prototyping and screen design tools, and they can be used to assess the interface as a whole.

Scenario

Scenario reduces both the number of features and the level of functionality. It can simulate the user interface as long as the user follows a previously planned path, i.e., a user can use a specific set of computer facilities to achieve a specific outcome under specified circumstances.

Scenarios can be easy and cheap to build, and to be used during early evaluation of a user interface design to get user feedback without the expense of constructing a running prototype. It can also be used for user testing if they are developed with slightly more detail than a pure narrative.

The concepts of vertical, horizontal and scenario prototyping are illustrated in Figure 7 [6].

                Figure 7: Two Dimensions of Prototyping

Computer-based prototypes become easier to implement than before because there are more and more software prototyping tools, such as  RAPID [1], HyperCard [1], CHIRP [12]. Besides, there are many easy-to-learn and easy-to-use 4th generation languages, such as Smalltalk and Microsoft Visual Basic^(R).
 

  Wizard of Oz

This is a method of testing a system that does not exist. It allows designers to test ideas without implementation a system.

The Wizard of Oz technique works as follows: the user interacts with a screen, but instead of a piece of software responding to the user's requests, a developer (the wizard) is sitting at another screen (generally in another room) simulating the system's intelligence and interacting with the user [1]. The wizard may simulate all or part of the system function. When setting up a Wizard of Oz simulation, experience with previously implemented systems is helpful in order to place realistic bounds on the wizard's "abilities" [10].

Early study of Wizard of Oz was the "listening typewriter" [5] simulation of a speech recognition interface where the user's spoken input was typed into a word processor by a human typist located in another room [6]. Wizard of Oz is usually used for the design of intelligent system [10], the experiment and development of natural-language recognition system [16], and the simulation of the system function which is difficult to implement in a prototype.

Wizard of Oz is ideal for the testing of preliminary prototypes and to gather users' expectations on the system. With this technique, the developers can develop a limited functionality prototype and enhance its functionality in evaluation by providing the missing functionality through human intervention without cost on programming. Extra understanding of the users can also be achieved through being involved so closely with the users.
 

  Slide shows and video prototyping

These techniques use the communication media to facilitate prototyping.

In slide shows, the storyboard prototype is encoded on a computer with software tools. The scene transition is activated by simple user input. The slides form a simple horizontal or vertical prototype.

Video prototyping eliminates software limitations. It requires no post-production editing or any special expertise in video production. The studies and experiments with video prototyping can be found in [17] and [18]. It is more like Muller's PICTIVE work [4], where multi-disciplinary design teams use it in informal brainstorming sessions. Unlike PICTIVE, video prototyping does not simply record design ideas, but create an evocative simulation of the proposed interface.

Here is an example of how to make video prototype for a pull down menu [17].  First, we draw a menu bar on paper and a mouse arrow on clear acetate. Second, turn on the camera and move the acetate so that it looks as if the mouse is moving over the menu bar. When the mouse is over the menu title, we make a clicking sound and pause the camera. In the third step, we draw the menu on a small piece of paper, put it under the menu title, and restart the camcorder. When viewing the tape, the menu appears to have been pulled down from the menu bar.

Both slide shows and video prototyping provide kind of simulation of the system. They appear to behave as a real system although they only show some scenes. The simulation is restricted by some tightly predefined tasks, and the user is hardly to interact with the system.
 
 

Table 2 gives a brief comparison of the medium-fidelity prototypings.

 
 

Techniques	Advantages	Disadvantages
Vertical	*test in depth under    realistic circumstances  *test with real user tasks	*only test a limited part    of the system
Horizontal	*test the entire interface  *can be implemented    fast	*can not perform real    tasks
Scenario	*easy and cheap to build	*only test a limited part    of the system  *can not perform real    tasks
Wizard of Oz	*save time on programming  *extra understanding of users can    be achieved by closely involved      with them	*people need to be    trained to act as    computers  *less realistic
Slide and video Simulation	*simple  *system simulation	*lack of flexibility  *users can not interact    with the system

 
                           Table 2:  Comparison of Medium-fidelity Prototyping Techniques

When to use prototyping techniques
    Prototyping is part of the design and development process, especially in the
    early stage.
    Figure 8 below [Dr. Saul Greenberg's lecture note] shows when and what
    kind of prototyping techniques are applied in the design life cycle.


                                                           Figure 8:  Design Process and Prototyping

Comparison of low-fidelity and medium(high)-fidelity prototyping [2]
 

Type	Advantages	Disadvantages
Low-Fidelity	*less time & lower cost  *evaluate multiple    design concepts  *useful communication    device  *address screen layout    issues	*limited usefulness for    usability tests  *navigational and flow    limitations  *facilitator-driven  *poor detailed    specification
Medium(high)-Fidelity	*partial/complete    functionality  *interactive  *user-driven  *clearly defines    navigational scheme  *use for exploration and    test  *marketing and sales tool	*time-consuming to    create  *inefficient for    proof-of-concept designs  *blind users to major    representaional flaws  *managements may think    it is real

 

Considerations when choosing prototyping techniques [2]

Although prototyping is recognized as an efficient way in interface design, the optimum methods of prototyping have not yet been agreed upon. There is a long time debate about which prototyping approach is better, low-fidelity or high-fidelity. The list below summarizes some of the key points to consider when deciding whether a low or medium(high)-fidelity prototyping approach would be most appropriate for your design and development needs.

  Cost and schedule constraints
If your budget and schedule are limited, you should first consider low-fidelity prototyping, especially paper mock-up, because they are very cheap and fast to develop. If there are experienced programmers and you have fast tools to build a computer-based prototype, medium-fidelity prototyping is also a consideration. High-fidelity prototyping is not recommended because it is expensive to build.
 
Proof-of-concept
Low-fidelity prototyping is the most efficient way to test design ideas because it is concentrated on the concept evaluation of a design. Medium(high) prototyping will not help more, however,  you spend more time and money on a Medium(high)-fidelity prototype than on a low-fidelity one.
 
Navigation and flow
Medium-fidelity prototyping is good to simulate the system's interaction. In low-fidelity prototyping, storyboard can show the system's direction.
 
User driven or facilitator-driven
 
If you need a user-driven prototype, medium(high)-fidelity prototyping is recommended because users can directly interact with the prototype. Otherwise, if you need a facilitator-driven prototype, low-fidelity prototyping is the choice.
 
Look-and-feel the product
 
Medium(high)-fidelity prototyping can help users gain the feeling of how the product works. If using a low-fidelity prototype, you must be good at facilitating the prototyping process. You need to know clearly what the real system will work, and you need to show and explain to cycle to the users.
 
Usability test
 
Medium(high)-fidelity prototyping are good for usability tests because they provide a relatively realistic appearance and functionality which is close to the final product. The usefulness that low-fidelity prototyping provides is too limited to conduct an usability test.
 
What is your facilitation skill/programming skill
 
If there are people who have the expertise in human-computer interaction and facilitation, low-fidelity could be the choice because running a low-fidelity prototyping needs such experience. However, if there are people who are experienced programmers, and you also have fast tools for interface generation, medium(high)-fidelity prototyping could be considered. Even though generally medium(high)-fidelity prototyping needs more time than low-fidelity prototyping, the expertise in programming and efficient tools could reduce the time and energy.
 
  What development stage you are
 
Figure 8 on this web page shows the relationship of prototyping and product development.
If you are in the very early stage of the design, low-fidelity prototyping will be efficient to help you work out the design concepts with the users.
If you are in the medium stage, such as screen design and usability testing, medium-fidelity prototyping will be more useful.
 

The relationship of prototypes and final products is as follows:
Throw-away
Prototypes only serve to elicit users' reaction and to evaluate design ideas. Prototypes will be thrown away in the later development phases. Such prototypes must be created rapidly and cheaply. Otherwise it will be too expensive to do prototyping.
 
Incremental
 
Product is built as separate modules. Each module is prototyped and tested, then added to the final system.
 
Evolutionary
 
A prototype is built from low-fidelity to high-fidelity, incorporating design changes, Eventually the prototype becomes the final product.

The Performance of Prototyping on Some Attributes

Is it quick and cheap to do?
Low-fidelity prototyping is fast and cheap to do because it is paper-based and provides limited or no functionality. Medium(high)-fidelity prototyping is more expensive than low-fidelity because it is interactive and provides partial or full functionality. Coding and debugging is needed in a computer-based prototyping.
 
Does it provide for controlled study?
Low-fidelity prototyping is good for participatory design, but not for controlled study because of their inherent restrictions, i.e., they can not simulate the system interaction so that the users can not perform tasks without facilitator's instructions. Medium(high)-fidelity prototyping can simulate system interaction. However, the simulation is not very realistic, so that the controlled study based on prototypes will be less reliable.
 
Is it suitable for qualitative analysis?
Prototyping is suitable for qualitative analysis. Prototypes provide a way for users to react to the conceptual design of a system, or to interact with a simulation of a system. Qualitative methods such as introspection, direct observation, heuristic evaluation are commonly used in evaluation of the design. Comparing to the evaluation of a final product, qualitative results, such as users' feedback and discussion are important in evaluation of a prototype.
 
  Is it suitable for quantitative analysis?
Low-fidelity prototyping is not suitable for quantitative analysis. Low-fidelity prototypes are too conceptual for a quantitative study.
Medium(high)-fidelity prototypes, especially computer-based ones,  can be used to get quantitative results, such as the users speed of performing a task and the errors they made. But the results will not be very realistic based on prototypes.
 
Does it require special equipment?
Low-fidelity prototyping does not require special equipment. Video camera may be needed, for example, in the video prototyping and PICTIVE prototyping.
Prototyping software is needed for computer-based prototypes.
 
Does it require special personnel?
Paper prototyping generally requires a facilitator, who knows the application thoroughly, to demonstrate or to test the application. The user is dependent on the facilitator to respond to the user's commands to turn cards or advance screen to simulate the flow of the application. The expertise in application and human-computer interaction is required.
Computer-based prototyping requires people with expertise in programming.

Working Examples

Windows^(R) 95 user interface prototyping and usability testing [7]

The design process of Windows 95 is used here as an example because:
(1) it shows how to design and develop the user interface for a large commercial software
(2) we can see how prototyping helps the designers and developers in the both the design and evaluation process
(2) people are familiar with the interface of Windows 95, so that no domain knowledge of the product is needed

In the project, an iterative design was selected other than a traditional "waterfall" design. In a "waterfall" design, the design of the system is usually limited to a specification writing phase, and the usability testing typically occurs near the end of the process. In this project, people needed much more opportunity to create a design, try it out with users, make changes, and gather more user feedback.

The iterative design was divided into three major phases: exploration, rapid prototyping, and fine tuning. Paper or computer-based prototypes were used to try out design ideas and to gather usability data in the lab.
Figure 9 outlines the process.

Figure 9: Windows 95 Iterative Design Process

Exploration phase

In this phase, the purpose is to set design directions and to gather initial user data.

An early prototype of the desktop was developed, with which the users can interact with some functions the group concern about. The usability studies of the prototype desktop were conducted in the Microsoft usability lab. Questions addressed were sometimes very broad ("Do users like it"), sometimes very specific ("After ten minutes of use, do users discover drag and drop to copy a file?"). Data collected included: verbal protocol, time per task, number of errors, types of errors, and rating information.

Test results showed that the design ideas about consistency with Window 3.1 were wrong. New design direction must be set up.
 

Rapid iteration phase

To save time, the team decided not to change the paper specification. Instead, they let the prototypes and code serve as a "living" specification. It allowed them to iterate at top speed.

They chose the major areas of the product function for iterative design and testing. They mocked up a number if representations in Visual Basic and tested them with users of all experience levels. In some tests comprehensive Visual Basic prototype is used. Figure 10 and 11 are examples for part of their prototypes. Some function and outlook of the prototypes, such as the design of Task Bar, is developed in the final product in the process of iterative design and evaluation.

    Figure 10: "Plate" Visualization for Minimized Windows

    Figure 11: Task bar
 

Fine tuning phase

Once they had designed and tested all of the major areas of the product, they realized that they had to see how all of the pieces fit together. Summative lab testing and longitudinal field study based on beta version of Windows 95 were conducted.
 

Conclusion

Iterative design, using prototypes and the product as the specification, and the constant testing with users allowed them to explore many different solutions to problems quickly. They had essentially found that the prototype was a richer type of specification. A prototype invites richer feedback, because the reviewer has to imagine less about how the system would work.

In this example it is seen that computer-based medium(high) prototypes played an important role in the iterative design, both in the specification phase or in the design phase. Computer-based prototypes enabled the team to conduct usability testing. The final product evolved from the process of prototyping and testing.
 
 

Design of network scenario generation tool

The network scenario generation tool was a course project I did last year. Different from the example above, it was a small, non-commercial software. It is an user interface for ATM-TN, an ATM network traffic simulation program [8].

Low-fidelity paper prototyping was used since there were no time and necessity to develop a computer-based prototype for such a small tool.

In the meetings with the users, I sketched the interface on paper and described the flow of the operation. Since the users are computer experts, they had no difficulties to understand the sketched functions and how the interface will perform. The users actively participated in the prototype design by providing their feedback and comments. The users were more concerned about the tasks than the screen design.

Once we reached the agreement on the design, I began the implementation. In the early stage of the implementation, the users were also invited to make comments on the functions and outlook of the system.

1. Preece, J. et.al., (1994) Extract-Chapter 22: Envisioning Design, p.451-465; Extract-Chapter 27: Prototyping, p.537-565. Human-Computer Interaction, Addison-Wesley.

2. Rudd, J., Stern, K. and Isensee, S. (1996) Low vs. high fidelity prototyping debate.  Interactions 3(1), p.76-85, ACM Press.
 
 3. Dix, A., Finlay, J., Abowd, G., Beale, R. (1993) Extract-Chapter 5: The Design Process, p.147-189; Chapter 11: Evaluation Techniques, p.363-400. Human-Computer Interaction, Prentice Hall.
 
4.  Muller, M.J., (1991) PICTIVE: An exploration in participatory design. In Proceedings of the ACM Conference on Human Factors in Computing Systems, p.225-231, ACM Press.
 
5. Gould, J.D., Conti, J., Hovanyecz, T. (1983) Composing letters with a simulated listening typewriter. Communications of the ACM 26, 4(April), p.295-308.

6. Nielsen, J. (1993) Extract-Chapter 4.8: Prototyping. In Usability Engineering, p.93-101, Academic Press.

7.  Sullivan, K. (1996) The Windows 95 User Interface: A Case Study in Usability Engineering. Proceedings of ACM CHI'96 Conference on Human Factors in Computing Systems, V.1, p.473-480.

8. Unger, B.W., Covington, A., Gburzynski, P., Gomes, F., Ono-Tesfaya, T., Ramaswamy, S., Williamson, C., Xiao, Z.(1995) A High Fidelity ATM Traffic and Network Simulator. Processing of the Winter Simulation Conference, Washington D.C.

9. Rettig, M. (1994) Prototyping for tiny fingers. Communications of the ACM, 37(4), ACM Press.

10. Maulsby, D., Greenberg, S., Mander, R.(1993) Prototyping an Intelligent Agent Through Wizard of Oz. Proceedings of ACM INTERCHI'93 Conference on  Human Factors in Computing Systems, p.277-284.
 
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13. Muller, M.J. (1992) Retrospective on a Year of Participatory Design using the PICTIVE Technique. Proceedings of ACM CHI'92 Conference on Human Factors in Computing Systems.
 
14. Miller, D. S., Smith, J.G., Muller, M.J. (1992) TelePICTIVE: Computer-Supported Collaborative GUI Design for Designers with Diverse Expertise. UIST'92, Monterey, California.

15. Nielsen, J. (1992) Teaching Experienced  Developers to Design Graphical User Interfaces. Proceedings of ACM CHI'92 Conference on Human Factors in Computing Systems.
 
16. Dahlback, N., Jonsson, A., Ahrenberg, L. (1993) Wizard of Oz Studies -- Why and How. Proceedings of the 1993 International Workshop on Intelligent User Interfaces p.193-200.

17. Young, E., Greenlee, R. (1992) Participatory Video Prototyping Posters: Helping Users, Programmers, and Designers. Proceedings of ACM CHI'92 Conference on Human Factors in Computing Systems -- Posters and Short Talks p.28.

18. Tognazzini, B. (1994) The "Starfire" Video Prototype Project: A Case History. ACM CHI'94 Conference on Human Factors in Computing Systems v.1 p.99-105.