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Designing useful and attractive products requires skill and creativity. As products evolve from initial ideas through conceptual design and prototypes, iterative cycles of design and evaluation help to ensure that they meet users' needs. But how do evaluators decide what and when to evaluate? The HutchWorld case study in the previous chapter described how one team did this, but the circumstances surrounding every product's development are different. Certain techniques work better for some than for others.

Identifying usability and user experience goals is essential for making every product successful, and this requires understanding users' needs. The role of evaluation is to make sure that this understanding occurs during all the stages of the product's development. The skillful and sometimes tricky part of doing this is knowing what to focus on at different stages. Initial requirements get the design process started, but, as you have seen, understanding requirements tends to happen by a process of negotiation between designers and users. As designers understand users' needs better, their designs reflect this understanding. Similarly, as users see and experience design ideas, they are able to give better feedback that enables the designers to improve their designs further. The process is cyclical, with evaluation playing a key role in facilitating understanding between designers and users.

Evaluation is driven by questions about how well the design or particular aspects of it satisfy users' needs. Some of these questions provide high­level goals to guide the evaluation. Others are much more specific. For example, can users find a particular menu item? Is a graphic useful and attractive? Is the product engaging? Practical constraints also play a big role in shaping evaluation plans: tight schedules, low budgets, or little access to users constrain what evaluators can do. You read in chapter 10 how the HutchWorld team had to plan its evaluation around hospital routines and patients' health.

Experienced designers get to know what works and what doesn't, but those with little experience can find doing their first evaluation daunting. However, with careful advance planning, problems can be spotted and ways of dealing with them can be found. Planning evaluation studies involves thinking about key issues and asking questions about the process. In this chapter we propose the DECIDE framework to help you do this.

The main aims of this chapter are to:

• Continue to explain the key concepts and terms used to discuss evaluation.
• Describe the evaluation paradigms and techniques used in interaction design.
• Discuss the conceptual, practical, and ethical issues to be considered when planning evaluation.
• Introduce the DECIDE framework to help you plan your own evaluation
  studies.