Fitts's Law - Success and Implications

Success and Implications

Fitts's law is an unusually successful and well-studied model. Experiments that reproduce Fitts's results and/or that demonstrate the applicability of Fitts's law in somewhat different situations are not difficult to perform. The measured data in such experiments often fit a straight line with a correlation coefficient of .95 or higher, a sign that the model is very accurate.

Although Fitts only published two articles on his law (Fitts 1954, Fitts and Peterson 1964), there are hundreds of subsequent studies related to it in the human–computer interaction (HCI) literature, and quite possibly thousands of studies published in the larger psychomovement literature. The first HCI application of Fitts's law was by Card, English, and Burr (1978), who used the index of performance (IP), defined as 1⁄_b, to compare performance of different input devices, with the mouse coming out on top. This early work, according to Stuart Card's biography, "was a major factor leading to the mouse's commercial introduction by Xerox".

Fitts's law has been shown to apply under a variety of conditions, with many different limbs (hands, feet, head-mounted sights, eye gaze), manipulanda (input devices), physical environments (including underwater), and user populations (young, old, special educational needs, and drugged participants). Note that the constants a, b, IP have different values under each of these conditions.

Since the advent of graphical user interfaces, Fitts's law has been applied to tasks where the user must position a mouse cursor over an on-screen target, such as a button or other widget. Fitts's law models both point-and-click and drag-and-drop actions. Dragging has a lower IP associated with it, because the increased muscle tension makes pointing more difficult. Fitts's law has also been shown to model target-directed hand and head motions in a virtual environment.

In its original and strictest form:

• It applies only to movement in a single dimension and not to movement in two dimensions (though it is successfully extended to two dimensions in the Accot-Zhai steering law);
• It describes simple motor response of, say, the human hand, failing to account for software acceleration usually implemented for a mouse cursor;
• It describes untrained movements, not movements that are executed after months or years of practice (though some argue that Fitts's law models behaviour that is so low level that extensive training doesn't make much difference).

If, as generally claimed, the law does hold true for pointing with the mouse, some consequences for user interface design include:

• Buttons and other GUI controls should be a reasonable size; it is relatively difficult to click on small ones.
• Edges and corners of the computer monitor (e.g., the location of the Start button and Taskbar in Microsoft Windows, and the menus and Dock of Mac OS X) are particularly easy to acquire with a mouse, touchpad or trackball because the pointer remains at the screen edge regardless of how much further the mouse is moved, thus can be considered as having infinite width. This doesn't apply to touchscreens, though.
• Similarly, top-of-screen menus (e.g., Mac OS) are sometimes easier to acquire than top-of-window menus (e.g., Windows OS).
• Pop-up menus can usually be opened faster than pull-down menus, since the user avoids travel: the pop-up appears at the current cursor position.
• Pie menu items typically are selected faster and have a lower error rate than linear menu items, for two reasons: because pie menu items are all the same, small distance from the centre of the menu; and because their wedge-shaped target areas (which usually extend to the edge of the screen) are very large.

Fitts's law remains one of the few hard, reliable human–computer interaction predictive models, joined more recently by the Accot-Zhai steering law, which is derived from Fitts's law.