On the Validity of Throughput as a Characteristic of Computer Input

Throughput (TP) has been a fundamental metric in quantifying input system performance. TP is a
concept based on Fitts’ law, an essentially two parameter (a, b) relationship between movement time (MT) and Fitts’ index of difficulty (ID). In part thanks to ISO 9241-9, the final draft international standard
(FDIS) of "Ergonomic requirements for office work with visual display terminals - Part 9: Requirements
for non-keyboard input devices", research and testing of computer input system in recent years have
increasingly relied on TP as the sole measure of performance quality of input devices. The goal was to standardize studies onto one metric that can be generalized and compared across different experimental studies. Unfortunately TP as defined in ISO 9241-9 is an ill-defined concept that changes with the mean index of difficulty used in measuring it and therefore cannot be generalized beyond specific experimental settings. Furthermore, important properties can be hidden when TP is used as a single metric to input system evaluation. We reason that it is more informative to use (a, b) parameters in Fitts’ law as separated metrics of an input system. One related issue, the foundation for post-hoc target size adjustment, known as effective width, is also discussed.