Further to my post about the downloadable usability.gov guidelines, I've picked out the bits from the chapter on 'Usability Testing' that are relevant to my work but it's worth reading the whole of the chapter if you're interested. My comments or headings are in square brackets below.
"Generally, the best method is to conduct a test where representative participants interact with representative scenarios.
The second major consideration is to ensure that an iterative approach is used.
Use an iterative design approach
The iterative design process helps to substantially improve the usability of Web sites. One recent study found that the improvements made between the original Web site and the redesigned Web site resulted in thirty percent more task completions, twenty-five percent less time to complete the tasks, and sixty-seven percent greater user satisfaction. A second study reported that eight of ten tasks were performed faster on the Web site that had been iteratively designed. Finally, a third study found that forty-six percent of the original set of issues were resolved by making design changes to the interface.
Participants tend not to voice negative reports. In one study, when using the ’think aloud’ [as opposed to retrospective] approach, users tended to read text on the screen and verbalize more of what they were doing rather than what they were thinking.
[How many user testers?]
Performance usability testing with users:
– Early in the design process, usability testing with a small number of users (approximately six) is sufficient to identify problems with the information architecture (navigation) and overall design issues. If the Web site has very different types of users (e.g., novices and experts), it is important to test with six or more of each type of user. Another critical factor in this preliminary testing is having trained usability specialists as the usability test facilitator and primary observers.
– Once the navigation, basic content, and display features are in place,
quantitative performance testing ... can be conducted
[What kinds of prototypes?]
Designers can use either paper-based or computer-based prototypes. Paper-based prototyping appears to be as effective as computer-based prototyping when trying to identify most usability issues.
Use inspection evaluation [and cognitive walkthroughs] results with caution.
Inspection evaluations include heuristic evaluations, expert reviews, and cognitive walkthroughs. It is a common practice to conduct an inspection evaluation to try to detect and resolve obvious problems before conducting usability tests. Inspection evaluations should be used cautiously because several studies have shown that they appear to detect far more potential problems than actually exist, and they also tend to miss some real problems.
Heuristic evaluations and expert reviews may best be used to identify potential usability issues to evaluate during usability testing. To improve somewhat on the performance of heuristic evaluations, evaluators can use the ’usability problem inspector’ (UPI) method or the ’Discovery and Analysis Resource’ (DARe) method.
Cognitive walkthroughs may best be used to identify potential usability issues to evaluate during usability testing.
Testers can use either laboratory or remote usability testing because they both elicit similar results.
Use severity ratings with caution."