fitts' law basics
If you can't read please download the document
TRANSCRIPT
Fits' Law Basics
Fitts Law BasicsLucas RizoliCPSC 544, September 2006
Paul Fitts
1954
Image from http://www.psychology.ru/whoswho/Paul_Fitts.stm
Serial tapping task
Image from http://www.tele-actor.net/fitts/fitts_background.html
Fitts Law
Fitts Law
a: Intercept b: Slope A: Amplitude W: Width ID: Index of difficulty
Target AreaWA
Time
Index of difficulty
Intercept
Slope (ms/bits)
Index of performance
Bits/ms Bandwidth Comparable across devices/tasks
Limits of Fitts
Strange results with small AOne-dimensionalPointing only
Fitts original ID
Multiplied by 2 to avoid negative ID Problematic when A < W/2
Better fit to data than Fitts original Possible negative ID
Welfords ID
Claude Shannon
1948
Image from http://www.daviddarling.info/encyclopedia/S/Shannon.html
Info Capacity
Info Capacity
Shannon formulation
Best fit to data Positive ID Follows from Info Theory
WTarget AreaA
HWTarget AreaA
WTarget AreaA
HWTarget AreaA
Fitts in two dimensions
Image from MacKenzie, I. S. and Buxton, W. 1992. Extending Fitts' law to two-dimensional tasks.
What to use as W? Status-quo (W) Smaller-of (min(H,W)) Approach (W) Perimeter (H + W) Area (H*W)
Fitts in two dimensions
Image from MacKenzie, I. S. and Buxton, W. 1992. Extending Fitts' law to two-dimensional tasks.
What to use as W? Status-quo (W) Smaller-of (min(H,W)) Approach (W) Perimeter (H + W) Area (H*W)
Appeal of Smaller-of
Significantly better than mostSimpler than W
W:H v. MT
Image from Accot, J. and Zhai, S. 2003. Refining Fitts' law models for bivariate pointing.
Bivariate pointing
Implications of BP Law
Third empirical parameterIdeal W:H ratio for rect. areasDirectional stability v. landing
Accot & Zhai
1997
Image from Accot J. and Zhai S. 1997. Beyond Fitts' law: Models for trajectory-based HCI tasks.
Application of steering
Evaluate non-pointing tasks Performance on changing path
Image from Accot J. and Zhai S. 1997. Beyond Fitts' law: Models for trajectory-based HCI tasks.
Questions?
Difficult target?Fastest location?Most forgiving target area?