Probing and its Effects on the Validity and Reliability of Verbal Reports
http://www.abolrous.com/sally/projects/probing_paper.htm

Abolrous, Sally
University of Washington-Seattle
2001

Abstract:

Eliciting verbal reports from participants in usability studies is a commonly used method used to collect performance and preference data. By asking users to 'think-aloud,' usability practitioners can observe users interact with an interface and listen to their concurrent thoughts at the same time. Verbal data is helpful because it allows observers to know how users think—what they look for, how they expect to accomplish tasks, and what elements of the interface they find confusing or helpful.