The Perils Of Measurement
http://infocentre.frontend.com/infocentre/articles/theperilsofmeasurement.html

Farrell, Tom
Frontend Infocentre
2001

Abstract:

Most 'usability' practitioners quite rightly put great emphasis on the ease with which sample users can complete pre-selected tasks. After all, it makes sense to ensure that common tasks, identified after research into user needs, should be completed as efficiently as possible. It is when this 'efficiency' is measured that problems can begin.