Fitts’ Law Fitts’ Law Rob Diaz-Marino Rob Diaz-Marino
Dec 22, 2015
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