The Hunt for Usability: Tracking Eye Movements
http://portal.acm.org/citation.cfm?id=632716.632823
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  peer-reviewed

Karn, Keith S., Steve Ellis and Cornell Juliano
Conference on Human Factors in Computing Systems
1999

Abstract:

Usability testing methods have not changed significantly since the origins of the practice. Usability studies typically address human performance at a readily observable task-level, including measures like time to complete a task, percentage of participants succeeding, type and number of errors, and subjective ratings of ease of use. Certain types of questions are difficult to answer efficiently with these techniques.