Fitts' Law

Paul Fitts found that, for simple pointing tasks, task completion time is proportional to the index of difficulty (IoD), which is essentially the logarithm of the distance to target divided by the target size: IoD = log2 ( distance to target/target size ). While Fitts' original research was on physical one-dimensional pointing alternating between two metal strips, Fitts’ Law has been found to hold for many kinds of pointing tasks, with a wide variety of devices, over multiple orders of magnitude. Although Fitts’ Law holds in many situations it is not universal; crucially it depends on situations where there is hand–eye coordination and where movement error is proportional to distance moved.