Fitts' Law

Question 1

Fitts Law

Answer 1

▪ Developed by Fitts in 1954
▪ Calculates the time it takes to perform actions such
as targeting a screen object
▪ Can be derived from, and explained with a simple model of our input movements.
▪ Fitt’s Law can be used to determine the size and location of a screen object

Question 2

Fitts’ Law Formula

Answer 2

▪ Fitts’s law gives us the relationship between the time it takes a pointer to move to a particular target in order to interact with it in some way

▪ T = a + b * log2(2D/W)

• a and b vary depending on the type of pointer

▪ a defines the intersection on the y axis and is often interpreted as a delay

▪ b is a slope and describes an acceleration

• the equation includes a logarithm which means that the time doesn’t grow linearly with the parameters but at a slower pace. So if something is twice far away it does take longer to reach out but not twice as long. That is because movement first accelerates and then decelerates.

Question 3

Fitt’s Law Model

Answer 3

▪ Targets with the same relative size concerning the movement length are reached in the same amount of time.
▪ The speed of movement doesn’t alter the time if the relative precision remains constant.
▪ Targets farther away and smaller in size necessitate multiple moves for precision: one to approach the target and others for accurate placement.

Question 4

Effective size calculation

Answer 4

For basic shapes like squares or circles, it’s their cross-section.

• For general objects, it’s the largest or closest square/circle that can fit inside.
• At screen boundaries, like the Apple-style menu at the top, objects might effectively appear larger in size since movement gets clipped at the boundary.

Question 5

Two component model

Answer 5

Any goal directed movement can be decomposed into two components:

• an initial movement: rapid and relatively coarse
• a final movement which is slower and intended to secure accuracy

Question 6

Fitts’s Law application to UX

Answer 6

The implications of Fitts’s law can be grouped into 2 distinct categories, corresponding to the two variables that affect the movement time:

• Target size: Make targets big
• Distance to target: optimise distance to target.