Introduction
The ultimate purpose of an interactive program is to improve the efficiency
and effectiveness of a user's performance, where performance
is a measure of the time and effort expended to complete tasks.
To improve performance we must know both how to measure it and what
factors affect or determine it. With this knowledge, we can measure
current performance, manipulate the identified determinants and then measure
the performance again to confirm that improvement has occurred.
User performance may be described in terms of a number of determinants,
including
