Summarize Participant Characteristics - I

This section of the test plan describes the characteristics of the end user(s) of the product/document that you will be testing. It is Important to work closely with others in your organization to determine the characteristics of the target users. For detailed procedures on how to establish the user profile and acquire participants, see Chapter 7. A basic example of participant characteristics for a usability test of a hotel reserv ations web site appears in Figure 5-3.

One thing to remember when describing the participant characteristics is to use the right number of participants. When it comes to selecting the number of participants to employ for a test, the overriding guideline is "You cannot have too many participants." When thinking about achieving statistically valid results, small sample sizes lack the statistical power to identify significant differences between groups. For a true experimental design, you must use a minimum of 10 to 12 participants per condition. However, for the purpose of conducting a less formal usability test, research has shown that four to five participants who represent one audience cell will expose about 80 percent of the usability deficiencies of a product for that audience, and that this 80

percent will represent most of the major problems. Of course, if you have the time and resources to study more than four or five participants, by all means do so. It is possible that the additional 20 percent of deficiencies you might find could be important for your product,

We have conducted many tests that held true to the preceding principle. In one, Jeff tested eight participants and discovered about 80 percent of the problems within the first four participants. However, participant 8, the last one, performed a particularly grievous error on one task that would have required a servicecall for the product. This would never have been uncovered