Source code for FittsLawApplet (html version)

Fitts' law is a robust model of human psychomotor behavior developed in 1954. The model is based on time and distance. It enables the prediction of human movement and human motion based on rapid, aimed movement, not drawing or writing.

It seems intuitive that movement time would be affected by the distance moved and the precision demanded by the size of the target to which one is moving. Fitts discovered that movement time was a logarithmic function of distance when target size was held constant, and that movement time was also a logarithmic function of target size when distance was held constant.

Mathematically, Fitts' law is stated as follows: 

MT = a + b log2(2A/W) 


MT = movement time 
a,b = regression coefficients 
A = distance of movement from start to target center 
W = width of the target

Move your mouse over the gray area to explore Fitt's Law. At each, you will see information predicted by Fitt's Law -- the time to reach the target (starting from that point), the distance to the center of the target, and the width of the target along the axis of movement toward the target center.
You can click on the target and drag it around the gray area, in order to investigate how the estimates change based on the location of the target. You can also click on the "resize" button to randomly change the size of the target, and thus be able to investigate how Fitt's Law estimates change with the size of the target.

Further Questions

1. With the target positioned in the top-left, find all the locations where the time to target is 300ms. Intuitively, you might think that these would be located on a circular arc, rotating around the center of the target. But this is NOT the case. Why?

2. Show the bands where the time to target is a constant 200ms, 300ms, 400ms.

3. If the mouse is already within the target area, what is the time to target? (Note: this is not displayed by the applet) Why does this value make sense?

Created by Andrew Freed, on February 15, 2002.
Modified by Andrew Freed, on April 15, 2002.

Development of this applet was sponsored by the Penn State Fund for Excellence in Learning and Teaching (FELT), project "Java-based Teaching of Mathematics in Information Sciences and Technology", supervised by Frank Ritter and David Mudgett.