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Design activities begin once a set of requirements has been established. Broadly speaking, there are two types of design: conceptual and physical. The former is concerned with developing a conceptual model that captures what the product will do and how it will behave, while the latter is concerned with details of the design such as screen and menu structures, icons, and graphics. The design emerges iteratively, through repeated design­evaluation­redesign cycles involving users. For users to effectively evaluate the design of an interactive product, designers must produce an interactive version of their ideas. In the early stages of development, these interactive versions may be made of paper and cardboard, while as design progresses and ideas become more detailed, they may be polished pieces of software, metal, or plastic that resemble the final product. We have called the activity concerned with building this interactive version prototyping and construction.

There are two distinct circumstances for design: one where you're starting from scratch and one where you're modifying an existing product. A lot of design comes from the latter, and it may be tempting to think that additional features can be added, or existing ones tweaked, without extensive investigation, prototyping or evaluation. It is true that if changes are not significant then the prototyping and evaluation activities can be scaled down, but they are still invaluable activities that should not be skipped.

In Chapter 7, we discussed some ways to identify user needs and establish requirements. In this chapter, we look at the activities involved in progressing a set of requirements through the cycles of prototyping to construction. We begin by explaining the role and techniques of prototyping and then explain how prototypes may be used in the design process. Tool support plays an important part in development, but tool support changes so rapidly in this area that we do not attempt to provide a catalog of current support. Instead, we discuss the kinds of tools that may be of help and categories of tools that have been suggested.

The main aims of this chapter are to:

• Describe prototyping and different types of prototyping activities.
• Enable you to produce a simple prototype.
• Enable you to produce a conceptual model for a system and justify your
  choices.
• Enable you to attempt some aspects of physical design.
• Explain the use of scenarios and prototypes in conceptual design.
• Discuss standards, guidelines, and rules available to help interaction designers.
• Discuss the range of tool support available for interaction design