Is Display/Control Gain a Useful Metric for Optimizing an Interface?

Abstract
Two studies were conducted to determine the adequacy of display/control gain as a metric for characterizing a control-display interface consisting of a CRT display and a separate input device. The first study examined the effect of independent changes in the control input and the display output on cursor positioning performance with two input devices: a touch tablet and a trackball. The second study separated the effects of control amplitude, display amplitude, and display target width. Results indicate that display/control gain is not an adequate specification for performance. A combination of gain and Fitts' index of difficulty may be a useful predictor.