Index of Difficulty

The Index of Difficulty (ID) is the central quantity in Fitts' law that captures how hard a rapid aimed pointing movement is, computed as log₂(A/W + 1) in the Shannon formulation, where A is the amplitude (distance to the target) and W is the target width along the movement axis. It is measured in bits, and higher values mean harder tasks. ID is paired with the Index of Performance (or throughput, in bits/second) to compare input devices and interaction techniques on a task-independent scale. For accessibility researchers, ID is a useful lens for understanding why small or distant targets disproportionately slow and frustrate users with motor impairments, and for evaluating whether techniques like target expansion, area cursors, or goal crossing genuinely lower difficulty.