When Observing Users Is Not Enough: 10 Guidelines for Getting More Out of Users' Verbal Comments
http://www.uxmatters.com/MT/archives/000183.php

Peyrichoux, Isabelle
UXmatters
2007

Abstract:

One of the principles underlying usability testing is that observing a user perform a task provides more reliable information than simply asking the user how easy it would be to perform the task. By observing users, you can assess whether they are actually able to use a product. By asking them, you simply cannot.