A law used in the Keystroke Level Model for estimating the time taken by an operator to complete a computing task. Fitts Law is used to determine the time to get a mouse pointer over an on-screen area.

t = a + b * log2((D/S) + 1)

Where a and b are determined empirically (typically a = 0, b= 0.1), D is the distance to the area and S is the size of the area.

The Keystroke Level Model is used in user-interface analysis to predict how long a task will take by considering the duration of each tiny step, such as a keystroke, or, as here, a mouse movement.

In lay terms, the bigger the target and the closer to your starting point, the faster you can get your pointer to it.

Fitts Law is cited by interface designers as justification for making "hot spots" fairly large and keeping links that are expected to be used in sequence relatively close together. Teeny tiny links widely separated cause user frustration. This is a bad thing for software and web site design.

Fitts's Law (pronounced "fitzez law") simply states the time to acquire a target is a function of the distance to and size of the target.

Fitts's Law applies mainly to user interface design. The simplest button to click is the closest and largest one. As an example of the first principle, right click menus are very efficient, because the distance between the target and the cursor is zero.

The latter principle is demonstrated by windowed applications. A windowed application has a buffer zone between the left toolbar and the edge of the screen. This buffer is the window's trim. When the user moves their mouse to click a tool, they are forced to slow down so they will not overshoot their target. By making the application full screen, the tool buttons will increase their size to include the edges of the screen. The user can then shoot their mouse to the corners and click without slowing down. Since the mouse will not run past the screen, the buttons have an effectively infinite area to the left.

Fitts's Law is generally one of the most important and ignored concepts of GUI design.

Some would express that the size of a target on a computer screen can be considered infinitely large if the button or link is touching an edge of the screen or one of the mouse pointer's boundaries. In such a case, the user may easily move their cursor toward the desired link, overshoot the input to their hearts' content, and still click the link without trouble. This makes for easy access to links with less time spent on aiming.

Luna, the Windows XP GUI, is likely the first instance of Windows to consistently adhere to this design principle in its taskbars. I, for one, find it effective and useful.

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