Fitt's Law (circa 1954) states that the time to acquire a target is a function of the distance to the target and the size of the target. The further the target is away from the user's current position, the longer it takes to move to the target. The smaller the target, the more the user has to use fine muscle control and hence take more time to move.The implications of Fitt's law varies with design field; what we have learned over the past 20 years only somewhat applies to mobile devices. Some interpretations of Fitt's Law:
- Mouse driven interfaces (software) — the "large" controls are the edges of the screen, as they are really infinitely large in one direction. Corners are larger still. Thus frequently used items should go around the edges. The existence of a cursor gives a precise definition of "close," so contextual menus can be truly context driven.
- Mouse driven web sites — when a link is activated, the screen changes, possibly completely, and the edges of the screen are not accessible by the web page. Thus "where the cursor is" is the largest target, and cultural visual scanning practices are used to place most elements. Consistency between pages helps the visual scanning process. Note: modern web development techniques allow for an interaction style more closely resembling software.
- Stylus driven interfaces (small screens) — the concept of "distance" is almost meaningless, as the entire screen is smaller than the hand and there is no cursor. Thus size and predictability of location become the key issues for speed of target acquisition.
- Scroll-and-select interfaces (small screens) — the number of keypresses to access a target is a good measure of distance, and size is reasonably represented by whether the target is currently displayed or not. As more devices display several font sizes, target size will be a combination of visibility and target size.