One of the most well-understood and salient principles underlying the ergonomics of graphical user interface design is Fitts' Law.
Named for Paul Fitts, a psychologist at Ohio State University, Fitts' Law is a mathematical model of fine motor control which predicts how long it takes to move from one position to another as a function of the distance to and size of the target area. Papers outlining what became known as Fitts' Law were published in 1954 and 1964.
Fitts himself was an expert in aviation psychology, and he developed his research around more ergonomic layouts for cockpit instrumentation as a way of increasing aviation safety. You can read more about the early history and mathematics behind Fitts' Law on Wikipedia.
Fitts' model proved especially relevant to the early research on computer input devices performed in the late 1970s. Although Fitts' model was originally formulated to project how quickly a human could point at a physical button, it turns out that the same set of rules governs how quickly someone can target an area on the screen with a mouse cursor.
Although there's a great deal of subtlety to Fitts' research, what became known as Fitts' law is a fairly simple intuitive concept.
- The farther away a target is, the longer it takes to acquire it with the mouse.
- The smaller a target is, the longer it takes to acquire it with the mouse.
The inverse of both statements is true as well (closer and bigger targets can be more quickly acquired.)
One common mathematical formulation of this relationship is:
- MT is the average time taken to acquire the target.
- a and b are empirical constants determined through linear regression.
- A is the distance from the starting point to the center of the target.
- W is the width of the target measured along the axis of motion (how close to the target you need to get to count as acquiring it.)
- c is a constant which is either 0, .5, or 1, depending on the specific environment.
Here's a cool Java-based applet which lets you play around with Fitts' Law to see how it feels in practice: http://www.tele-actor.net/fitts/
How Fitts' Law Affects User Interface
The key takeaway for interface designers is clear: the farther away a button is from the current mouse position, the larger it needs to be to achieve the same average acquisition speed. Put another way, there are two main ways to improve mouse efficiency: put the controls closer, or make them bigger. (There are other more avant-garde ways to alter the physics of mouse travel which I won't go into today.)
Over the years, as monitors have gotten bigger and screen resolutions have increased, Fitts' law dictates that actual mouse efficiency has gone down.
Think about Word 1.0, which was designed for a common maximum 640x480 screen resolution. Toolbar buttons in Word 1.0 were 20x20 buttons with 16x16 icons in them.
Word 2003, on the other hand, is commonly run at resolutions as high as 1600x1200 and beyond--yet the toolbar buttons remain the same 20x20 size they were in Word 1.0. But because the screen is so much larger, most of the time your mouse cursor will be much farther away than it could have been on a 640x480 screen. Greater mouse distances mean an increased MT target acquisition time.
In other words, the same button takes much longer to click than it did fifteen years ago.