Getting the Complete Picture with Usability Testing
http://www.usability.gov/pubs/030106news.html

Bailey, Robert
Usability.gov
2006

Abstract:

Good usability testing definitely provides an opportunity for clear-cut improvements in the usability of Web sites. In order to increase the chances of success with usability testing, it is important to measure effectiveness, efficiency, and satisfaction–they all measure different aspects of the usability of a Web site. If only one or two of these measures are used, it would provide an incomplete or partial picture of the possible human performance and user satisfaction results.