Grounding in the Basics of User-Centered Design

Grounding in the basics of human information processing, cognitive psychology, and user-centered design (essentially the domain of the human factors specialist) helps immensely because it enables the test moderator to sense, even before the test begins, which interactions, operations, messages, or instructions are liable to cause problems. Test moderators with this background have a knowledge of which problems can be generalized to the population at large

and which are more trivial. This helps to ascertain when to probe further and what issues need to be explored thoroughly during the debriefing session. 

Additionally, this background can also prevent the need to test situations that are known to cause problems for users, such as the inappropriate use of color or the incorrect placing of a note in a manual. Lastly, a strong background in usability engineering helps the test moderator to focus on fixing the important issues after a test is complete.