Paul M. Fitts modeled human psychomotor behavior by describing the phenomena of variability of a response as a function of response duration using information theory concepts. He introduced an index of difficulty of a movement which relates average values for amplitude, duration and variability of successive movements . He proposed a speed-accuracy trade-off using a principle of motor control where activities done quicker can be less accurate than those activities done slower.
ISO 9241-9 establishes uniform guidelines and testing procedures for evaluating computer pointing devices. The metric for comparison is Throughput, in bits per second (bits/s), which includes both the speed and accuracy of users' performance. The equation for throughput is Fitts’ Index of Performance except using an effective index of difficulty (ID). Specifically,
Throughput = ID / MT (1)
where MT is the mean movement time, in seconds, for all trials within the same condition, and
ID = log2(D / W + 1) (2)
ID, in bits, is calculated from D, the distance to the target, and W, the effective width of the target. W is calculated as
W = 4.133 × SD (3)
where SD is the standard deviation of the end-point positions using observed end-point scatter data. Using effective width allows throughput to incorporate the spatial variability in human performance. It includes both speed and accuracy .