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A central aspect of interaction design is user testing. User testing involves measuring the performance of typical users doing typical tasks in controlled laboratory­like conditions. Its goal is to obtain objective performance data to show how usable a system or product is in terms of usability goals, such as ease of use or learnability. More generally, usability testing relies on a combination of techniques including observation, questionnaires and interviews as well as user testing, but user testing is of central concern, and in this chapter we focus upon it. We also examine key issues in experimental design because user testing has developed from experimental practice, and although there are important differences between them there is also commonality.

The last part of the chapter considers how user behavior can be modeled to predict usability. Here we examine two modeling approaches (based on psychological theory) that have been used to predict user performance. Both come from the well­known GOMS family of approaches: the GOMS model and the Keystroke level model. We also discuss Fitts' Law.

The main aims of this chapter are to:

• Explain how to do user testing.
• Discuss how and why a user test differs from an experiment.
• Discuss the contribution of user testing to usability testing.
• Discuss how to design simple experiments.
• Describe the GOMS model, the Keystroke level model and Fitts' law and discuss when these techniques are useful.
• Explain how to do a simple keystroke level analysis.