1.
Fitts’ Law Basics Lucas Rizoli CPSC 544, September 2006

2.
Paul Fitts 1954 Image from http://www.psychology.ru/whoswho/Paul_Fitts.stm

3.
Serial tapping task Image from http://www.tele-actor.net/fitts/fitts_background.html

4.
Fitts’ Law

5.
Fitts’ Law a : Intercept b : Slope A : Amplitude W : Width ID : Index of difficulty

6.
W A

7.
Time Index of difficulty Intercept Slope (ms/bits)

8.
Index of performance Bits/ms Bandwidth Comparable across devices/tasks

9.
Limits of Fitts’ Strange results with small A One-dimensional Pointing only

10.
Fitts’ original ID Multiplied by 2 to avoid negative ID Problematic when A < W/2

11.
Better fit to data than Fitts’ original Possible negative ID Welford’s ID

12.
Claude Shannon 1948 Image from http://www.daviddarling.info/encyclopedia/S/Shannon.html

13.
Info Capacity

14.
Info Capacity

15.
Shannon formulation Best fit to data Positive ID Follows from Info Theory

16.
W W Target Area A H A

17.
W W Target Area A H A

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.
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.
Appeal of Smaller-of Significantly better than most Simpler than W ´

21.
W:H v. MT Image from Accot, J. and Zhai, S. 2003. Refining Fitts' law models for bivariate pointing.

22.
Bivariate pointing

23.
Implications of BP Law Third empirical parameter Ideal W:H ratio for rect. areas Directional stability v. “landing”

24.
Accot & Zhai 1997 Image from Accot J. and Zhai S. 1997. Beyond Fitts' law: Models for trajectory-based HCI tasks.

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.
Questions?

27.
Difficult target? Fastest location? Most forgiving target area?