Fitts’ LawFitts’ Law

Rob Diaz-MarinoRob Diaz-Marino

OverviewOverview The BasicsThe Basics

Who invented it?Who invented it? What does it model?What does it model? How is it used in HCI?How is it used in HCI?

Fitts’ AdaptationFitts’ Adaptation The FormulaThe Formula DrawbacksDrawbacks A Standardized TestA Standardized Test

An ExampleAn Example SynthesisSynthesis ConclusionConclusion

Who invented it?Who invented it?

Paul Fitts (in 1954)Paul Fitts (in 1954)

Psychologist at Ohio State UniversityPsychologist at Ohio State University

US Air ForceUS Air Force Aviation safetyAviation safety

What does it Model?What does it Model?

Info Capacity of Human Motor systemInfo Capacity of Human Motor system

Ability to guide pointing device to targetAbility to guide pointing device to target Coarse movementCoarse movement Overshoot / UndershootOvershoot / Undershoot AdjustmentAdjustment

Based on Shannon’s Theorem 17Based on Shannon’s Theorem 17

How is it used in HCI?How is it used in HCI?

Predictive FormulaPredictive Formula

Rapid aimed motion with a Pointing DeviceRapid aimed motion with a Pointing Device Mouse, trackball, stylus, finger (on touch screen) etc.Mouse, trackball, stylus, finger (on touch screen) etc.

EstimatesEstimates Time to acquire targetTime to acquire target Difficulty of acquiring targetDifficulty of acquiring target

Shannon’s Theorem 17Shannon’s Theorem 17

Formula (From CPSC 441)Formula (From CPSC 441) Maximum effective transmission capacityMaximum effective transmission capacity Applied in NetworkingApplied in Networking

Capacity = Maximum effective transmission capacity for a medium

Bandwidth = Physical transmission capacity

Signal Power = Strength of the signal being carried

Noise Power = Strength of noise interference over the medium

Log base 2 because transmitting binary (2 signal levels: 0 and 1)

Fitts’ AdaptationFitts’ Adaptation Human Motor System = MediumHuman Motor System = Medium Human Perception = ChannelHuman Perception = Channel Target Distance = SignalTarget Distance = Signal Target Width = NoiseTarget Width = Noise Bandwidth = Index of PerformanceBandwidth = Index of Performance

Log base 2?Log base 2?

The FormulaThe Formula

Movement Time (s)

(constant)

Minimum reaction time (s)

Index of Difficulty

(bits)

Amplitude Width

(constant)

Index of Performance

(Time/bit)

VariationsVariations

Welford (1960; 1968, p. 147)

MacKenzie (1989, p. 324)

Amplitude and WidthAmplitude and Width

A

W

Amplitude and WidthAmplitude and Width

A

W

Amplitude and WidthAmplitude and Width

A

W

DrawbacksDrawbacks

Formula doesn’t always workFormula doesn’t always work Ex. For easy tasks, sometimes MT < 0Ex. For easy tasks, sometimes MT < 0

Not always accurateNot always accurate Different people, different “bandwidths”Different people, different “bandwidths” Learning effectsLearning effects

Drawbacks (2)Drawbacks (2)

Different pointing devices, different timesDifferent pointing devices, different times Mouse vs. TrackballMouse vs. Trackball

Solution:Solution: Adjust with constants a & bAdjust with constants a & b

A Standardized TestA Standardized Test

ISO 9241-9: Picture from Gutwin (2002)

Regression line used on empirical results to determine a and b.

An ExampleAn Example

Which interface is more efficient for browsing?Which interface is more efficient for browsing?

An ExampleAn Example

Which interface is more efficient for browsing?Which interface is more efficient for browsing?

An Example (2)An Example (2)

A = 25cmW = 2cm W = 2cm

From previous experiments with a mouse: a = 230ms, b = 166 ms/bit

An Example (3)An Example (3)

A = 5cm

W = 2cm W = 2cm

From previous experiments with a mouse: a = 230ms, b = 166 ms/bit

An Example (3)An Example (3)

Difference of 357.3 msDifference of 357.3 ms Worst case – Delete every 2Worst case – Delete every 2ndnd photo photo 35.73 sec / 100 photos35.73 sec / 100 photos 5 min 57 sec / 1000 photos5 min 57 sec / 1000 photos Is redesign worth it?Is redesign worth it?

SynthesisSynthesis

To reduce target acquisition time:To reduce target acquisition time: Decrease target distanceDecrease target distance Increase target sizeIncrease target size Consider direction of approachConsider direction of approach

ConclusionConclusion

Cheap quantitative metricCheap quantitative metric Judges GUI design & efficiencyJudges GUI design & efficiency Not exact – rough estimateNot exact – rough estimate Applicable to pointing devicesApplicable to pointing devices Models “hand-to-eye” coordinationModels “hand-to-eye” coordination