Fitts’s lawis a model of human movement primarily used in human–computer-interaction that predicts that the time required to rapidly move to a target area is a function of the distance to the target and the size of the target. For a given target acquisition task, e.g. pointing to an object on computer, Fitts’ law desribes how the distance from start point to the target and the width of the target influence

Shannon Formulation of the Fitts’ law:

Ais the task’s amplitude or distance(distance between point A to B)

W is the target’s width,

ID is the aforementioned Index of Difficulty.

From this formula we can see that the task gets harder, the farther away the target is and the smaller it is. This fits well with the common perception of such tasks and, as it turns out, the time it takes to perform such a task is linearly correlated with its ID.

As an assistant student of the couse "Human Computer Interaction 2",  I made a small Fitts’s law web test using the Javascript library D3.  By moving the mouse from one circle to another,  you can see from the scatter graph that the ID varies during the time.

Try to click the red circle as fast as possible but at the same time try to avoid errors. You can change the ciecle radius and distances between each circle at the right side

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