When You Shouldn’t Use Fitts’s Law To Measure User Experience (article summary)


"The key statement of Fitts's Law is that the time required to move a pointing device to a target is a function of the distance to the target and its size. In layman's terms: the closer and larger a target, the faster it is to click on that target. This is easy to understand, not too difficult to implement and it doesn’t seem to make much sense to contradict such a simple and obvious statement.

However, before you start applying Fitts's Law on every single pixel you can find, consider a few problems that might arise for you as an interaction designer."

Fitts's Rule Number 1: Create Larger Targets

The likely most prominent statement derived from Fitts’s Law is that the larger a target, the faster it is to acquire.

This can be accomplished by utilizing every available pixel to enlarge the clickable area and thus make it a larger target to click. However, considerations about available screen real estate must be taken into consideration. Also, there is a point of diminishing return where the benefit of increasing the clickable area for a large button 10% is less beneficial than increasing the clickable area of a small button 10%.

Fitts's Rule Number 2: Minimize Cursor Movement

A second statement one can deduce from Fitts's Law is that the closer a target, the faster it is to acquire.

This can be accomplished by placing frequently used buttons close together reducing the amount of pixels for a cursor to travel. However, doing so could cause a conflict with other important design principles, such as the principle of grouping and separating different classes of functionality or content. Another principle Fitts's Law can interfere with is the principle of providing a clean and tidy interface. A third, very important principle that may force you to defy Fitts's Law is the principle of building forgiving interfaces, which aim to prevent and minimize the costs of mistakes.  The consequences of mistakes are less severe when the elements represent navigational functionality as opposed to sharing or editing functionality.

Fitts's Rule Number 3: Avoid Muscular Tension

The goal of Fitts's index of performance (PDF) is to quantify the information capacity of the human motor system. In other words: it aims to rank input methods according to the amount of physical effort they require to execute a certain command.

High decision input methods are used to make sure a command is not executed accidentally.

A second reason to implement more awkward input methods is to take advantage of the space-efficient nature of gestures. According to Fitts's index of performance, gestures, which involve some degree of dragging, require a higher muscular tension, which is why Fitts’s Law favors pointing-and-clicking. However, the advantage of gestures is that they trigger functionality without requiring UI controls.

Fitts's Rule Number 4: Exploit The Prime Pixels

The concept of prime pixels states that some pixels are faster to acquire than others. Corners and edges of the screen are especially fast to access. However, the fastest-to-acquire pixel in any situation is simply the pixel at the current cursor position, which has lead to the introduction of the right-click context menu into human–computer interaction.

There are two kinds of context menus: linear menus and radial or pie menus. The pie menu, although favored by Fitts's Law, does have a few disadvantages that can outweigh its benefits in certain situations. One issue is that the circular menu shape quickly leads to small target areas when more menu entries are added. Circular menus take up more space.

As to the corners and edges of the screen, two potential problems should be mentioned when working with mouse-operated devices. On large screens, the amount of pixels the cursor will have to travel can somewhat offset the aforementioned benefits. Also, Web designers will not be able to benefit from this rule because their content (except when in full-screen mode) is run in a browser window. As a result, they cannot take advantage of the edges of the screen and will almost necessarily have to opt for a more compact, centered layout.

Final Thoughts

The difficulty faced by interaction designers and user experience designers is that they have to consider, balance and combine measurable and non-measurable dimensions of user experience to create the best possible product. Fitts's Law tries to help user interface designers by giving them easily quantifiable, mathematically accurate values to base their design decisions on.

Of course, it is often possible to measure the quality of an interface using mathematical values: The fewer clicks required to access a certain set of data, the faster the navigation. The more vertical pixels a horizontally aligned interface preserves, the better it is suited for the respective device orientation. The closer the most frequently accessed buttons are placed, the more economic the cursor movement.

However, since interfaces are designed for humans, they also have to be consistent, considerate, inclusive, playful and discoverable: qualities that can hardly be measured as easily as clicks or pixels. The stunning accuracy and simplicity of mathematical formulas may sometimes lead designers to favor the measurable over not-so-easily-measurable dimensions. And while mathematical formulas can, indeed, help you enrich user experience, you should treat those formulas as tools, not as principles.

Instead, you should debate and choose anthropological principles first, and, if they permit it, use formulas such as Fitts's Law as much as possible to actually improve user experience.