Fitts' Law: When It Doesn't Apply And Why

Fitts's Law is a predictive model of human movement that is primarily used in human-computer interaction and ergonomics. It predicts the time taken to move to and select a target, such as a button or link, based on the target's size and distance. However, there are certain scenarios where Fitts's Law may not apply or be suitable. For example, it is important to note that Fitts's Law is concerned with rapid, pointing movements and not continuous motion. Additionally, while the law suggests creating larger targets for easier interaction, this may not always be feasible due to screen space constraints or aesthetic considerations. Furthermore, placing elements closer together to minimise cursor movement may conflict with other design principles, such as grouping similar functionalities or creating a clean and tidy interface. Lastly, Fitts's Law may not be suitable for certain input methods, such as gestures or two-step input methods, which prioritise security and space efficiency over speed and accuracy.

Fitts's Law doesn't apply to continuous motion, such as drawing

Fitts's Law, a predictive model of human behaviour, does not apply to continuous motion, such as drawing.

Fitts's Law was developed by psychologist Paul Fitts in 1954. It states that the time taken for a person to move a pointer (e.g. a mouse cursor) to a target area depends on the distance to the target and its size. The law is applicable to rapid, pointing movements, not continuous motion.

Drawing involves continuous motion, which usually consists of one large movement followed by fine adjustments. Fitts's Law does not account for this type of movement. Instead, it is particularly relevant in visual interface design or any interface involving pointing, where it is used to determine the appropriate sizes of interactive elements and identify potential design usability problems.

For example, Fitts's Law has influenced the convention of making interactive buttons large, especially on finger-operated mobile devices. Smaller buttons are more challenging and time-consuming to click. Similarly, the distance between a user's task/attention area and a task-related button should be kept as short as possible.

By following Fitts's Law, standard interface elements such as right-click pop-up menus or short drop-down menus have been successful in minimising the user's travel distance and time, thereby increasing productivity.

However, when it comes to continuous motion like drawing, Fitts's Law is not applicable. This is because drawing involves a different set of movements and adjustments that fall outside the scope of rapid, pointing movements that Fitts's Law addresses.

It doesn't apply to children, as they exhibit different speeds and accuracy in motor tasks

Fitts's Law is a predictive model of human behaviour that describes the relationship between the speed and accuracy of human movement. It was developed by psychologist Paul Fitts in 1954 and is widely applied in user experience (UX) and user interface (UI) design.

Fitts's Law does apply to children, but due to developmental factors, they might exhibit different speeds and accuracy in motor tasks. Research has shown that when young children execute pointing tasks, they perform at levels below older children and adults. However, the foundational principle of Fitts's Law, relating movement, distance, and target size, remains consistent across age groups.

A study by Hourcade et al. compared the performance of 13 4-year-olds, 13 5-year-olds, and 13 young adults in point-and-click tasks. The study found that age had a significant effect on accuracy, target reentry, and Fitts's Law's index of performance. The results suggest that Fitts's Law models point-and-click tasks performed by 4 and 5-year-olds very well, but only when they first enter the target.

Another study by Raya et al. evaluated the suitability of two input devices, a standard computer mouse and a head mouse, for child-computer interaction. The study involved 10 children aged between 6 and 8 performing a multidirectional pointing task. The results showed that Fitts's Law can be used to detect changes in the learning process and assess the level of psychomotor development. The study also found meaningful differences between the fine motor control (hand) and gross motor control (head) when using the two devices.

In summary, while Fitts's Law does apply to children, they may exhibit different speeds and accuracy in motor tasks due to their developmental stage. This has implications for UX and UI design, and researchers suggest that interfaces should be designed specifically for preschool children to accommodate their unique needs.

It doesn't apply to two-dimensional tasks, as it was meant to apply to one-dimensional tasks

Fitts's Law is a predictive model of human movement that is commonly applied to two-dimensional target acquisition tasks on interactive computing systems. However, it is inherently a one-dimensional model. This means that when applied to two-dimensional tasks, the model can break down and yield unrealistic results.

The law predicts that the time required to move to a target area is a function of the ratio between the distance to the target and the width of the target. In other words, the longer the distance and the smaller the target's size, the longer it takes. This is easy to understand when applied to one-dimensional tasks, but when applied to two-dimensional tasks, it becomes more complex.

In two-dimensional tasks, the role of target width becomes unclear. For example, if the target is a rectangle, such as a word, and the approach angle is 90 degrees, the roles of width and height reverse from the perspective of the model. This means that the horizontal measurement is considered the target "width", regardless of the approach angle. This can lead to unusual interpretations of task difficulty.

To address this issue, researchers have proposed alternative interpretations of "target width" that accommodate the two-dimensional nature of tasks. One approach is to use the Shannon formulation, which always provides a positive (or zero) rating for the index of task difficulty. Another approach is to substitute for the target width a measure that is more consistent with the two-dimensional nature of the task, such as the width of the target along the approach vector or the smaller of the width or height.

These modifications to Fitts's Law allow for more accurate predictions of movement time and error rates in two-dimensional tasks. By taking into account the approach angle and target shape, researchers can extend the applicability of the law to a wider range of human-computer interactions.

It doesn't apply to eye tracking, as the user is blind during fast saccadic eye movements

Fitts's Law is a predictive model of human behaviour that is used in user interface design to determine the proper sizes of interactive elements, the best location for them, and to notice potential usability issues. It states that the amount of time required for a person to move a pointer to a target area is a function of the distance to the target divided by the size of the target.

Fitts's Law does not apply to eye tracking, as the user is blind during fast saccadic eye movements. Saccadic eye movements are rapid, ballistic eye movements that abruptly change the line of gaze. They are so fast that there is inevitable blurring of the image on the retina, as the retina is sweeping the visual field. This blurring is not useful, so the eye has a mechanism that "cuts off" the processing of retinal images when it becomes blurred. This phenomenon is called saccadic masking or saccadic suppression.

During saccadic eye movements, the user is blind, and therefore cannot see the target they are trying to reach. This means that Fitts's Law, which is based on the size of the target and the distance to the target, does not apply in this scenario.

It doesn't apply to temporal targets, which are defined purely on the time axis

Fitts' Law is a predictive model of human behaviour that is used to optimise user interactions with digital interfaces. It was created by psychologist Paul Fitts in 1954 and states that the time taken to move a pointer (e.g. a mouse cursor) to a target area depends on the distance to the target and its size. In other words, the longer the distance and the smaller the target, the longer it takes.

Fitts' Law is widely applied in user experience (UX) and user interface (UI) design. For example, it has influenced the convention of making interactive buttons large, especially on mobile devices with finger-operated screens. It is also used to determine the optimal size and location of interactive elements, and to identify potential usability issues.

However, Fitts' Law does not apply to temporal targets, which are defined purely on the time axis. This means that targets which are not dependent on distance or size are not covered by Fitts' Law.

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