# Tag Info

A model in HCI and ergonomics that correlates the time taken to target an object to the object's size and its distance away.

Fitts’ law (a.k.a “Fitts’s law” was proposed by psychologist Paul Fitts in 1954. Put simply, it predicts that an object that is larger and/or closer to the pointer (where pointer may refer either to a computer cursor, or to a physical object such as a finger or stylus) will take less time to target under rapid motion.

It has been mathematically described by Scott MacKenzie (the so-called Shannon formulation) thusly (taken from the Wikipedia article on the subject):

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

Where:

• T is the average time taken to complete the movement,
• D is the distance from the starting point to the centre of the target object,
• W is the width of the target, measured along the axis of motion, and
• a and b are arbitrary constants defined experimentally as representing the start/stop time of the device and the inherent speed of the device respectively.

Fitts’ law has two main complementary implications for interface design (both in software and in real-world interfaces):

1. Targets that the user will want to click on most often or most quickly (e.g. targets that trigger ‘save’ or emergency safety operations) should be relatively large, and relatively close to the likely starting position of the user's pointer (ideally directly underneath it), and
2. Targets the user will not want to click on, especially accidentally (e.g. targets that trigger destructive actions such as ‘delete’ functions) should be relatively small, and/or relatively far away from the likely starting position of the user's pointer.

Interestingly, Fitts’ law also means that items along hard edges and especially in corners are especially easy to target (being, essentially, infinitely wide in the direction of motion).

## Questions that are appropriate for this tag

• Ways to apply Fitts’ law to a given problem, e.g. by reducing the effective distance between the starting point and a particular target (i.e. D) or by increasing target size (e.g. by placing elements along screen edges)
• Ways to balance target time aesthetically with information hierarchy in context such that both are given appropriate consideration
• Questions about Fitts’ law itself, including ways to apply it in 2 or 3 dimensions where an item's "width" in the plane of motion may not be clear
• When it's appropriate to ignore the lessons of Fitts’ law.