Fitts' Law Basics

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The theory behind Fitts' well-known pointing law, commonly used in human-computer interaction. Also, some recent work in modelling users' pointing performance.

Presented in the Fall of 2006 for CPSC 544 (http://www.cs.ubc.ca/~cs544/Fall2006/)

Published in: Technology, Business

Fitts' Law Basics

  1. 1. Fitts’ Law Basics Lucas Rizoli CPSC 544, September 2006
  2. 2. Paul Fitts 1954 Image from http://www.psychology.ru/whoswho/Paul_Fitts.stm
  3. 3. Serial tapping task Image from http://www.tele-actor.net/fitts/fitts_background.html
  4. 4. Fitts’ Law
  5. 5. Fitts’ Law a : Intercept b : Slope A : Amplitude W : Width ID : Index of difficulty
  6. 6. W A
  7. 7. Time Index of difficulty Intercept Slope (ms/bits)
  8. 8. Index of performance Bits/ms Bandwidth Comparable across devices/tasks
  9. 9. Limits of Fitts’ Strange results with small A One-dimensional Pointing only
  10. 10. Fitts’ original ID Multiplied by 2 to avoid negative ID Problematic when A < W/2
  11. 11. Better fit to data than Fitts’ original Possible negative ID Welford’s ID
  12. 12. Claude Shannon 1948 Image from http://www.daviddarling.info/encyclopedia/S/Shannon.html
  13. 13. Info Capacity
  14. 14. Info Capacity
  15. 15. Shannon formulation Best fit to data Positive ID Follows from Info Theory
  16. 16. W W Target Area A H A
  17. 17. W W Target Area A H A
  18. 18. 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 )
  19. 19. 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 )
  20. 20. Appeal of Smaller-of Significantly better than most Simpler than W ´
  21. 21. W:H v. MT Image from Accot, J. and Zhai, S. 2003. Refining Fitts' law models for bivariate pointing.
  22. 22. Bivariate pointing
  23. 23. Implications of BP Law Third empirical parameter Ideal W:H ratio for rect. areas Directional stability v. “landing”
  24. 24. Accot & Zhai 1997 Image from Accot J. and Zhai S. 1997. Beyond Fitts' law: Models for trajectory-based HCI tasks.
  25. 25. 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.
  26. 26. Questions?
  27. 27. Difficult target? Fastest location? Most forgiving target area?

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