“Fitts’s law is a fundamental principle of human-computer
interaction that explains how people interact with digital interfaces. It states
that the time needed to move a pointer to a target region is directly related to
its distance and is inversely proportional to its size. To put it another way,
the larger the target and the closer it is to the cursor, the easier and faster
it is to hit it.”
Fitts’ law was suggested by psychologist Paul Fitts in 1954, who was interested
in the motor abilities required in human-computer interaction. Fitts’ law has
now become a cornerstone of user experience design, and it is essential in
building intuitive and easy-to-use interfaces.
The Formula of Fitts’s Law Fitts’s law is often expressed as a
mathematical formula:
T = a + b log2(2D/W)
Where T is the time required to move the cursor to the target, D is the distance
between the cursor and the target, and W is the width of the target. a and b are
empirically determined constants that depend on the user’s speed and accuracy.
According to the formula, the time required to reach a goal is a function of the
destination’s distance and size.
The logarithmic term represents the fact that the time necessary to reach a
destination reduces linearly as the target size grows. Instead, there is a
logarithmic relationship between goal size and time, which implies that doubling
the target size decreases the time necessary to achieve it by a constant amount.
Fitts’s law is most often used to describe pointing tasks, such as clicking on a
button or selecting a menu item. However, it can also be applied to other types
of interactions, such as scrolling or dragging.
Examples of Fitts’s Law in Action
Fitts’s law is everywhere in modern user interface design. Here are a few
examples of its usage:
1. Touchscreen Buttons
The size and placement of buttons on a touchscreen are critical factors in their
usability. Fitts’s law predicts that larger buttons that are closer to the
user’s finger will be easier to hit accurately and quickly. For example, in the
iOS interface for the iPhone, the home button is large and centered, making it
easy to hit with the thumb.
2. Scrollbars
Scrollbars are another interface element that benefits from Fitts’s law. By
making the scrollbar wider, it becomes easier to hit accurately and quickly,
reducing the time required to scroll through a long document. In many modern
interfaces, the scrollbar disappears when not in use, but reappears when the
user starts scrolling, making it easier to target.
3. Drop-Down Menus
Drop-down menus are often used in interfaces to provide access to additional
options or features. The size and placement of the menu are critical to their
usability. Fitts’s law predicts that larger menus that are closer to the cursor
will be easier to target. In the Windows operating system, for example, the
Start menu is located at the bottom left of the screen, making it easy to target
with the cursor.
Retention:
Measures the rate at which existing users are returning. Examples of retention
metrics include how many active users from a given time period are still present
in some later time period.
4. Video Game Interfaces
Fitts’s law is also important in video game interface design. In first-person
shooter games, for example, the player must be able to target adversaries with
the mouse swiftly and precisely.
Fitts’s law predicts that larger targets, such as enemy heads or torsos, will be
easier to hit accurately and quickly, reducing the time required to kill
enemies.
5. Web Design
Fitts’s law is also relevant in web design. For example, in navigation menus, the
size and placement of the links can greatly affect their usability. A larger
link that is placed closer to the cursor will be easier to click on, reducing
the time required to navigate to the desired page. This is why many current
websites have a huge navigation menu at the top of the page, making it simple to
target and click on links.
Fitts’s law can also be applied to the design of web forms. The size and
placement of form fields can greatly affect their usability. Fitts’s law
predicts that larger form fields that are closer to the cursor will be easier to
fill out accurately and quickly. This is why many modern web forms have large
text boxes and radio buttons, making them easy to target and fill out.
Challenges in Applying Fitts’s Law
While Fitts’s law is a powerful tool for designing interfaces that are easy to
use, there are some challenges to applying it in practice.
One of the main challenges is that the law assumes that the user is able to
accurately move the cursor to the target. However, in some cases, users may
have difficulty moving the cursor due to physical limitations such as a
tremor or arthritis. In these cases, Fitts’s law may not be applicable, and
alternative interface design strategies may need to be used.
Another challenge is that the law assumes that the user is able to perceive
the target accurately. In some cases, the target may be difficult to see due
to poor contrast or color choice. In these cases, the user may have
difficulty targeting the object, even if it is large and close to the
cursor.
Finally, Fitts’s law assumes that the user is motivated to complete the
task. In other circumstances, the user may lack the motivation to complete
the activity or may be distracted by external factors such as a loud
environment or competing requests.
In these cases, Fitts’s law may not be applicable, and alternative interface design
strategies may need to be used.
Conclusion
Fitts’s law is a fundamental principle of human-computer interaction that
explains how people interact with digital interfaces. It asserts that the time
it takes to move a cursor to a target region is directly related to its distance
and inversely proportional to its size.
Fitts’s law is used in the design of interfaces such as touchscreen buttons,
scrollbars, drop-down menus, video game interfaces, and web design.
However, there are challenges to applying Fitts’s law in practice, such as
physical limitations, perceptual difficulties, and motivational factors. Despite
these challenges, Fitts’s law remains a powerful tool for designing interfaces
that are intuitive and easy to use.
