Model
Fitts's law has been formulated mathematically in several different ways. One common form is the Shannon formulation (proposed by Scott MacKenzie, professor at York University, and named for its resemblance to the Shannon–Hartley theorem) for movement along a single dimension:
where:
- T is the average time taken to complete the movement. (Traditionally, researchers have used the symbol MT for this, to mean movement time.)
- a represents the start/stop time of the device (intercept) and
- b stands for the inherent speed of the device (slope). These constants can be determined experimentally by fitting a straight line to measured data.
- D is the distance from the starting point to the center of the target. (Traditionally, researchers have used the symbol A for this, to mean the amplitude of the movement.)
- W is the width of the target measured along the axis of motion. W can also be thought of as the allowed error tolerance in the final position, since the final point of the motion must fall within ±W⁄2 of the target's center.
From the equation, we see a speed–accuracy trade off associated with pointing, whereby targets that are smaller and/or further away require more time to acquire.
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