Moderating with Multiple Personalities: Three Roles for Facilitating Usability Tests
http://www.uie.com/articles/moderating_multiple_personalities

Spool, Jared M.
User Interface Engineering
2009

Abstract:

Usability tests are a core design tool and, when done well, they deliver tremendous insights to the team. However, when a usability test is done poorly, it can be a disaster for everyone involved. An important key to their success is the work of a great moderator.