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Fitts's Law

Fitts's Law

The time to acquire a target is a function of the distance to and size of the target.

This law especially applies to buttons, which the purpose of these elements is to be easy to find and select. Make elements you wish to be easily selectable large and position them close to users.
According to this law, fast movements and small targets result in greater error rates, due to the speed-accuracy trade-off. Although multiple variants of Fitts’ law exist, all encompass this idea. Fitts’ law is widely applied in user experience (UX) and user interface (UI) design. For example, this law influenced the convention of making interactive buttons large (especially on finger-operated mobile devices)—smaller buttons are more difficult (and time-consuming) to click. Likewise, the distance between a user’s task/attention area and the task-related button should be kept as short as possible.
Although originally developed according to movement in the physical world, in human-computer interaction Fitts's Law is typically applied to movement through the graphical user interface using a cursor or other type of pointer.

Applying Fitts’s Law to User Interface Design
The size of a target and its distance from the user’s current position within the user interface affect user experience in a number of ways. Some of the major implications for user interface design and user experience in turn are considered below:

1.       Command buttons and any other interactive element in the graphical user interface must be distinguished from other non-interactive elements by size. Whilst it may seem obvious, user interface design often ignores that the larger a button is the easier it is to click with a pointing device. As interactive objects decrease in size there is a smaller surface area, requiring a level of precision that increases selection times.

2.       The outer edges and corners of the graphical user interface can be acquired with greater speed than anywhere else in the display, due to the pinning action of the screen. As the user is restricted in their movements the pointing device cannot move any further when they reach the outermost points of the screen; fixing the cursor at a point on the periphery of the display.

3.       Pop-up menus better support immediate selection of interactive elements than dropdown menus as the user does not have to move the cursor from its current position. Therefore, graphical designs that allow the user to interact without moving help to reduce the 'travel time'.

4.       Selecting options within linear menus, whether vertical (e.g. dropdown menus) or horizontal (e.g. top-level navigation), takes longer than clicking options in pie menus - where choices are arranged in a circle. Travelling distance is the same for all options in pie menus, unlike linear menus where distance increases the further along or down the list of options the user goes. In addition, the size of target areas is large in the pie menu, with the wedge-shaped buttons affording a larger margin for error when moving the cursor.

5.       Task bars impede movement through the interface as they require a more time-consuming level of precision than when options are placed on the outer limits of the screen. Although unconnected to Fitts's Law, multiple task bars can introduce a certain level of confusion or at the very least require the user to engage consciously with the screen arrangement to ensure appropriate selection.
  

A Simple Rule
The aim of user interface design should be to reduce the distance from one point to the next and make the target object large enough to enable prompt detection and selection of interactive elements without sacrificing accuracy. One particular aspect of this is ensuring that users are able to click anywhere on an interactive element to carry out the assigned action. For example, links, whether they are in the form of text, images or buttons (such as those seen in the ASOS screenshot below), should afford clicking on the whole region.

In Summary
Fitts's Law is applied to the design of interactive objects in graphical displays. As the size of an object increases the selection time goes down and as the distance between the user's starting point and the object decreases so to does the time taken to make the selection. Conversely, small objects, placed far away from the user's starting position take longest to select. Therefore, designers should aim to make objects as close to one another when they are used in the same sequence chain. They should also try to make the interactive elements of the screen as large as is sensible with the amount of space available. Small objects spread apart take the longest time to select; avoid this combination of design features as much as possible.

References:
When You Shouldn’t Use Fitts’s Law To Measure User Experience Anastasios Karafillis (Smashing Magazine)
Human Factors and Fitts’ Law Ken Goldberg, IEOR and EECS (Slides)
https://www.interaction-design.org/literature/article/fitts-s-law-the-importance-of-size-and-distance-in-ui-design

https://www.youtube.com/watch?v=95RoKSFyQ_k
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