If anyone has ever said to you “The faster I go the behindeder I get” they are expressing a result of Fitts Law in action: The Speed – Accuracy Tradeoff.

Fitts law describes human motion limitation from the standpoint of speed and its effect on accuracy. Generally stated, the faster you need to make a response the less accurate you can be.

Fitts initial experiments involved pointing moving the hand from a marked position on a table to a point either on the desk or on a computer screen. The fingertip was the pointer. Fitts first reported his findings in a scientific journal in 1954. Since then, numerous experiments has shown it to be true. In fact, all research has shown that the accuracy of the mathematical predictions are 94% accurate.

Some people are faster than others. Some people are more accurate. The formula still works out. Although the original formula was developed focusing on unlearned movements, it seems to hold true even after training occurs. The proportion of speed vs accuracy remains the same. Human capability for accuracy has definable limits.

Fitts Law is one of the few hard, reliable human–computer interaction predictive models. Mathematically, Fitts Law (modernized formula proposed by Scott MacKenzie, professor of York University, and named for its resemblance to the Shannon–Hartley theorem) is described as:

T = a + b log2 (1 + D/W)

• T is the average time taken to complete the movement.
• a represents the start/stop time of the movement while b stands for the inherent speed. (These constants can be determined experimentally by fitting a straight line to measured data [e.g. averages]. The constants change depending on the pointing device used. A mouse and stylus may both be used for pointing, but have different constants a and b associated with them.)
• 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. This is essentially the target size.

The average time it takes to complete the movement is determined by a combination of speed, the distance of the target point from the starting point and the size of the target. The speed of movment determines how accurate you can be.

Fitts Law and Mousing

Studies have shown that Fitts Law applies to all mouse action: point and click, drag and drop, etc.

If you’ve ever tried to mouse to small targets, you can appreciate how Fitts law applies to using the computer. A webpage or program may have many small targets for your mouse. It will slow down your work. The designer may be looking at another tradeoff.

Often, designers feel they need to include a large number of options to the customer. The greater the number of choices that need to be compressed into limited space, the smaller the target has to be…It isn’t that they don’t know about Fitts Law. They have other concerns. From their viewpoint the small size and somewhat slower work pattern may be worth it. Work-arounds have been menu bars, drop down menuse, pop-ups, etc.

Designing games is another question. There, speed is most important. The targets are generally fairly large and easy to find. The designer may also enlarge the target area so that it includes not just the middle of the object, but some space around it as well. A larger target changes the Speed – Accuracy tradeoff. Speed is rewarded while accuracy is less important.

Fitts Law and Everyday Life

If you ever get into a situation where accuracy is a problem, just remember – enlarge the target or slow down. Threading a needle is one good example. If the needle has a large eye the job goes much quicker. If the eye is smaller, approaching it with the thread slowly does help.

Threading a needle brings up other human limitations.

It is difficult to maintain accuracy in joints that are far from the center of the body unless those that are close to the center are stabilized. So, to get the accuracy you need for small eyed needles you must stabilize the shoulder, elbow and wrist. Movement comes primarily from the hand and fingers.

There are unrelated problems, such as difficulty seeing the target, or the target and the pointer being very close to the same size (this is true for needles and thread as well as shoelaces and grommet holes. When the target and the object are very close in size and the entire pointer end needs to be within the target area, there is a high need for accuracy.

Everyone in every task that requires accuracy experiences the Speed / Accuracy tradeoff. It’s part of life. Recognizing it when accuracy problems occur may help in finding a solution to your frustration.

Just take it easy, slow down, and try again.

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