Fitts' Law Discussion Exam Questions

1) What components of a target acquisition task make up the index of difficulty in Fitts' Law? Describe how variations to these components affect movement time.

The components of a target acquisition task that make up the index of difficulty are the amplitude of the movement (the distance from the start position to the center of the target) and the target width. Variations to these components affect movement time in the following way: as amplitude increases it takes longer to move to a target; similarly, as target width decreases it also takes longer to land on target. So, big targets at close range are acquired faster than small targets at long range.

2) What is meant by index of difficulty in context of Fitts' Law?

The index of difficulty (ID) provides a measure of the difficulty of a motor task. The ID is a simple number measured in bits that takes both movement distance and target width into account.

3) Describe two different methods for dealing with width irregularities in angular approaches to a target in a Fitts task.

Two different methods for dealing with width irregularities in angular approaches are the Smaller-Of model and the Area model. The Smaller-Of model interprets the width as the smaller of the height or width of the target. The Area model interprets the width as the target width times the target height.

4) Why are negative index of difficulties theoretically unsound? How have researchers dealt with this problem?

A negative index of difficulty is unsound because it implies that there is no difficulty involved in a motor task and furthermore it implies a negative movement time. Researchers have dealt with this problem by using Shannon's formulation in calculating ID, or by using a different interpretation for target width.

5) Under what circumstances does Fitts' law yield unrealistically low or even negative values for a task's index of difficulty (ID).

Fitts' law often yields low or negative ID values when applied to two-dimensional target acquisition tasks. The model breaks down because of problems in interpreting target width. In addition, when the amplitude of a target is very small the ID can become very low.

6) What is positioning time in Fitts' law?

Positioning time in Fitts' law is the time measured from when cursor movement starts until the selection button is pressed.

7) Explain why Fitts' law prediction equations have seen little use in design tools.

Researchers often come up with different prediction equations for the same device. Naturally, the question arises as to which equation is correct. So, incorporating Fitts' law prediction equations into design tools will remain just a thought until research can validate the measures found.

8) Describe any two situations (not necessarily user interface related) where Fitts' Law could be applied, one serial and one discrete.

A serial task could involve assembly line work. Reaching for parts and then placing them in their places as the line moves is a serial event. A discrete task is word-processing, where a pointing device (such as a mouse) is used to select things, but in between each usage of the pointing device, the user types on the keyboard.

9) Give an example of how Fitts' Law can be incorporated into user interface evaluation software.

A designer puts together a series of screens, their buttons, menus, icons, etc. Then the designer works the interface by positioning, selecting, and doing a series of typical operations. At this point, the embedded Fitts' law model produces predicted performance times for the tasks. The designer can then move, change or scale objects and screens and then request a reanalysis of the same task.

10) Will extending Fitts' Law to three dimensions, help in the evaluation of 3D input devices? (Magellan (space-puck), dataglove, etc.)

Extending Fitts' law to three dimensions will help in the evaluation of 3D input devices.