Card Sorting: Pushing Users Beyond Terminology Matches (Jakob Nielsen's Alertbox)
http://www.useit.com/alertbox/word-matching.html

It's easy to bias study participants, whether in user testing or in card sorting, if they focus on matching stimulus words instead of working on the underlying problem.

"I usually say user testing is easy: basically, you get some real customers and watch them use your site or app. But this article touches on one of the difficulties of running great studies: minimizing bias. To achieve this, you have to see how people behave on their own rather than impose your own thinking on them. In the latter case, they simply echo it back, and you don't learn how to improve your design for real-life use...."

how to (and how not to) pose the problem in useability testing