Fitts' Law

Fitts' Law is a mathematical model of human movement that was proposed in a 1954 paper by pyschology researcher Paul Fitts. The law is now a commonly known principle in UX design, as it predicts the difficulty of users clicking on certain UI elements. The law expesses the time required to move a pointer to a target as a function of the distance to the target and the size of the target:

D is the distance to the target, W is the size of the target, T is time, and ID is the difficulty index of a specific movement to a target. The law predicts that time is a linear function of difficulty index, which you can check for yourself by clicking on the circles below and building a plot of your data points.