Fitts' Law

Question 1

What is Fitts’ Law?

Answer 1

A model to predict how long it takes a user to move between two points and hit a target.

It predicts that the time taken is proportional to the distance and width of the target.

It is one-dimensional.

Question 2

What is Fitts’ Law’s formula(e)?

Answer 2

MT = a + bID
where a is the intercept
where b is the slope

where ID = log2(D+W/W)
where D is distance
where W is width

Question 3

What is the index of difficulty?

Answer 3

ID is a measure of how difficult it is to hit a target of a certain width from a certain distance.

Question 4

What is throughput in Fitts’ ?

Answer 4

The number of bits of information a user can communicate per second independently of a specific target.
TP = 1/b
or
TP = IDbar/MTbar

Question 5

How do you minimise MT?

Answer 5

Minimise D or maximise W

Question 6

What are the assumptions of Fitts’ Law?

Answer 6

• assumes closed loop interaction
• only models biologically plausible distances
• a and b vary depending on the task

Question 7

What are the limitations of Fitts’ Law?

Answer 7

• only low level actions are modelled reliably