Fitts's Law: Pointing Models, Measurement, and Interface Design

All flashcards

What does Fitts's law predict?

It predicts the time needed to point to and acquire a target as a function of distance to the target and its width.

What are $D$ and $W$?

$D$ is the distance from the starting position to the target center. $W$ is the target width along the movement axis and can be understood as the available error tolerance.

What is the original index of difficulty?

Fitts's original index is $ID=\log_2(2D/W)$. It represents the difficulty of a pointing task in bits.

What is the Shannon formulation?

It is the widely used form $ID=\log_2(D/W+1)$, proposed by Scott MacKenzie.

What is throughput?

Throughput is index of difficulty divided by movement time: $TP=ID/MT$. It is commonly expressed in bits per second.

Quiz questions

1. Why does applying Fitts's law to two-dimensional targets require choosing a target-width measure?Answer: The classic formulation was intended for one-dimensional pointing tasks.
2. Which model is more appropriate for navigation through a constrained path, such as a menu path?Answer: The steering law.
3. In temporal pointing, what does temporal width $W_t$ represent?Answer: The duration for which a target is available.
4. If distance and target width are both multiplied by the same factor, what happens to the basic index of difficulty?Answer: It stays the same because the distance-to-width ratio is unchanged.
5. Why can screen edges make edge-anchored targets easier to acquire?Answer: The pointer cannot move past the edge, making the target effectively long along that axis.