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Fitts' Law
Law of Steering
Hick's Law
Exercise 3: Law of Steering (2/2)

Fitts' Law

Fitts' Law

What is Fitts' Law?

Paul M. Fitts (1954): The Information Capacity of the Human Motor System in Controlling the Amplitude of Movement, Journal of Experimental Psychology, 47, 381-391.
model of human movement
predicts the time required for a rapid, aimed movement to a target area
example: moving a mouse pointer over a button
originally applied to one-dimensional movement, extended by various researchers
time needed for movement depends on distance and width of target

Fitts' Law

MT = a + b * log₂(A/W + 1)

MT = Movement Time
a, b = empirically determined constants (depending on task)
A = amplitude (distance)
W = target width
log₂ term also called index of difficulty (ID)

+1 was added later (Shannon form)
better fits empirical results and never lets ID get below zero

Defining Target Width

Target width originally for 1D targets. How is it defined for 2D targets?

W = width
W = (width + height) / 2
W = diameter of inscribed circle
W = width * height
W = weighted combination of width and height (Accot and Zhai, 2003

Law of Steering (Steering Law)

steering law

Accot and Zhai (1997): "Beyond Fitts' Law: Models for Trajectory-Based HCI Tasks", Proceedings of CHI 1997, ACM
derived from Fitts' Law
predicts time required to move a pointer through a 'tunnel'
time depends on length and width of the tunnel

Law of Steering (Steering Law)

MT = a + b * ( A / W )
for straight tunnels of constant width

MT = a + b * ∫_C ds / W(s)
for curved tunnels with varying width W(s)

MT = movement time

a,b = empirically determined constants (depending on task)
A = length of tunnel
B = width of tunnel

Consequences for UI design

Frequently used actions should get larger buttons
Increasing the size of a small widget helps much more than increasing that of a large one
Frequently used actions should be close to the cursor position (or vice versa)
Screen borders are targets with unlimited width - no overshooting possible

"Start"-Button on Windows, task bar, close button
Max OS X: menu bar at top of screen

Hick's Law (Hick-Hyman Law)

Predict time for making a choice between n options of equal probability

T = b * log₂(n+1)

Example: pick the one red card between black cards
works well for pre-attentive processing
does not work if the user has to consciously process all options first (linear time)

Sources & Recommended Reading

Flash tutorial on Fitts' Law
I. Scott MacKenzie: Fitts' Law as a Research and Design Tool in Human-Computer Interaction