Success Rate: The Simplest Usability Metric
Jakob Nielsen, Alertbox: "In addition to being expensive, collecting usability metrics interferes with the goal of gathering qualitative insights to drive design decisions. As a compromise, you can measure users' ability to complete tasks. Success rates are easy to understand and represent usability's bottom line." (2001-02-18)
The link address is: http://www.useit.com/alertbox/20010218.html