Note: This web page is really old, and not being maintained any more, but I've left it up because it may be of use to some.

Generalized Fitts' Law Model Builder

The Generalized Fitts' Law Model Builder (GFLMB) project started in late 1994 with an attempt to build a tool for performing Fitts' law studies of mouse movement. The tool that we (Scott MacKenzie and William Soukoreff) produced seemed to us to be useful to other researchers doing similar research, so we made the GFLMB software available to the research community.

The most recent version of the GFLMB is version 1.1, which was released in March 1996.
This page last changed: January 29th, 1997.

Fitts' Law (A Very Brief Tour)

Fitts' law is a relation derived from information theory (specifically Shannon's Theorem 17) which very successfully models human movement. It was first described by Fitts in 1954.

Fitts' law models rapid, aimed, movements, where one appendage (like a hand) starts at rest at a specific start position, and moves to rest within a target area. Fitts' law is written:

MT = ID / IP

where MT is the movement time, the time required to complete the motion; ID is the index of difficulty of the task (defined below); and IP is the index of performance. The index of performance is a constant for a specific appendage. The movement time is commonly measured in milliseconds, the index of difficulty in bits, and the index of performance in bits per second.

The index of difficulty was originally defined by Fitts (1954) as

ID = log2( 2A / W )

where A is the amplitude of the movement (the distance from the start position to the centre of the target); and W is the width of the target. Both A and W are measured in units of distance (millimetres).

Other formulations for the index of difficulty have been proposed. For example there is the Welford formulation (1960)

ID = log2( A / W + 0.5 )

currently, the preferred formulation is that proposed by MacKenzie (1989)

ID = log2( A / W + 1 )

This is the preferred formulation because it: always yields a positive index of difficulty, provides a slightly better fit with empirical data than the other formulations, and because it exactly reflects Shannon's Theorem 17, which Fitts' law is based upon.

The GFLMB software allows the experimenter to investigate the application of Fitts' law to the manipulation of a computer input pointing device (such as a mouse). It has been established that Fitts' law provides an accurate and efficacious means to predict the movement time of a pointing device being manipulated by a human operator.

Why the GFLMB?

Many researchers have investigated Fitts' law, applying it to a wide range of movement tasks. Each of these researchers have had to develop their own software to present movement tasks to subjects, and to accurately record the results. The GFLMB software is a highly-configurable Fitts' law experiment system, that only needs to be configured to be used for a specific experiment. The GFLMB provides complete control over the amplitude of motion, the dimensions and shape of the target, as well as many other aspects of the movement task. All of the flexibility and control provided by GFLMB is available through a simple-to-use configuration screen.

As well as being a research tool, the GFLMB is also useful to educators faced with teaching HCI. The GFLMB allows students to experiment with Fitts' law, and to do their own studies for course projects.

Resources Available Online

The GFLMB is provided free to the research community for use and distribution. This software is Copyright (C) 1996 by William Soukoreff and Scott MacKenzie. (A complete description of what you may and may not do with the software is included in the manual.)

The GFLMB software is available via ftp at:

You will recieve a copy of the manual in text form when you ftp the GFLMB package from the address given above. The manual is also available in HTML format at:

GFLMB Mailing List

A mailing-list does exist for those who would like to be informed about upcoming releases of the GFLMB software. If you would like your e-mail address to be added to this list, then please send e-mail to William Soukoreff () with a subject line of "GFLMB Mailing List". You should receive an acknowledgement within a day or so.