The problem of consistency in the design of Fitts' law experiments: consider either target distance and width or movement form and scale

ABSTRACT

An intriguing anomaly of the usual way of designing Fitts' law experiments in experimental psychology and HCI is exposed: experiments are traditionally designed so as to carefully balance two ancillary factors, target distance D and target width W, but not task difficulty, the factor unanimously thought to be crucial. Troubling factor confounds and hence quantitative estimation errors result from this inconsistency. The problem, it is suggested, may be fixed if the equivocalness of the fractional expression D/W that appears on the right-hand side of Fitts' law equations is acknowledged. This two-degree-of-freedom expression can be taken to specify either D and W or the form F and the scale S of the movement task. The paper ends up with practical recommendations for the design of consistent Fitts' law experiments. In most cases eliminating one factor will allow a safer estimation of Fitts' law parameters, while simplifying the experimental work.

REFERENCES

• 1: Abrams, R.A., Kornblum, S., Meyer, D.E.,&Wright, C.E. (1983). Fitts' law: Optimization of initial ballistic impulses for aimed movements. Bulletin of the Psychonomic Society, 22, 335.
• 2: Card, S.K., English, W.K.,&Burr, B.J. (1978). Evaluation of mouse, rate controlled isometric joystick, step keys and text keys for text selection on a CRT. Ergonomics, 21, 601--613.
• 3: Elliott, D., Helsen, W.F.&Chua, R. (2001). A century later: Woodworth's two-component model of goal-directed aiming. Psychological Bulletin, 127, 342--357.
• 4: Fitts, P.M. (1954). The information capacity of the human motor system in controlling the amplitude of movement. Journal of Experimental Psychology, 47, 381--391 (reprinted in Journal of Experimental Psychology: General, 1992, 121(3), 262--269).
• 5: Fitts, P.M.,&Peterson, J.R. (1964). Information capacity of discrete motor responses. Journal of Experimental Psychology, 67, 103--112.
• 6: Guiard, Y. (1993). On Fitts' and Hooke's laws: simple harmonic movement in upper-limb cyclical aiming. Acta Psychologica, 82, 139--159.
• 7: Yves Guiard , Michel Beaudouin-Lafon, Target acquisition in multiscale electronic worlds, International Journal of Human-Computer Studies, v.61 n.6, p.875-905, December 2004 [doi>10.1016/j.ijhcs.2004.09.005]
• 8: Yves Guiard , Renaud Blanch , Michel Beaudouin-Lafon, Object pointing: a complement to bitmap pointing in GUIs, Proceedings of Graphics Interface 2004, p.9-16, May 17-19, 2004, London, Ontario, Canada
• 9: Yves Guiard, Disentangling relative from absolute amplitude in Fitts' law experiments, CHI '01 extended abstracts on Human factors in computing systems, March 31-April 05, 2001, Seattle, Washington [doi>10.1145/634067.634254]
• 10: Yves Guiard, De l'ambiguïté des écritures fractionnaires: comment interpréter l'expression D/W dans la loi de Fitts?, Proceedings of the 19th International Conference of the Association Francophone d'Interaction Homme-Machine, November 12-15, 2007, Paris, France [doi>10.1145/1541436.1541440]
• 11: Yves Guiard , Michel Beaudouin-Lafon, Preface: Fitts' law 50 years later: Applications and contributions from human-computer interaction, International Journal of Human-Computer Studies, v.61 n.6, p.747-750, December 2004 [doi>10.1016/j.ijhcs/2004.09.003]
• 12: Hoffmann, E.R. (1996). The use of dimensional analysis in movement studies. Journal of Motor Behavior, 28 (2), 113--123.
• 13: ISO. ISO/DIS 9241-9 Ergonomic requirements for office work with visual display terminals (VDTs) - Part 9: Requirements for non-keyboard input devices. International Standard, International Organization for Standardization (2000).
• 14: Jagacinski, R.J., Repperger, D.W., Moran, M.S., Ward, S.L.,&Glass, B. (1980). Fitts' law and the microstructure of rapid discrete movements. Journal of Experimental Psychology: Human Perception and Performance, 6, 309--320.
• 15: Langolf, G.D., Chaffin, D.B.,&Foulke, J.A. (1976). An investigation of Fitts' law using a wide range of movement amplitudes. Journal of Motor Behavior, 8, 113--128.
• 16: Luce, R.D. (2003). Whatever happened to information theory in psychology? Review of General Psychology, 7, 183--188.
• 17: I. Scott MacKenzie, Fitts' law as a research and design tool in human-computer interaction, Human-Computer Interaction, v.7 n.1, p.91-139, March 1992 [doi>10.1207/s15327051hci0701_3]
• 18: MacKenzie, I.S.,&Soukoreff, R.W. (2002). Text entry for mobile computing: Models and methods, theory and practice. Human-Computer Interaction, 17, 147--198.
• 19: McMahon, T.A.& Bonner, J.T. (1983). On Size and Life. New York. Scientific American Library.
• 20: Meehl, P.E. (1997). The problem is epistemology, not statistics: Replace significance tests by confidence intervals and quantify accuracy of risky numerical predictions. In L.L. Harlow, S.A. Mulaik, and J.H. Steiger (Eds.), What if there were no significance tests? (pp. 393--425). Mahwah, NJ: Erlbaum. Available online at http://www.tc.umn.edu/~pemeehl/169ProblemIsEpistemology.pdf
• 21: Meyer, D.E., Abrams, R.A., Kornblum, S., Wright, C.E.,&Smith, J.E.K. (1988). Optimality in human motor performance: Ideal control of rapid aimed movements. Psychological Review, 95, 340--370.
• 22: Plamondon, R.,&Alimi, A.M. (1997). Speed/accuracy trade-offs in target-directed movements. Behavioral and Brain Sciences, 20, 279--349.
• 23: Schmidt, R.A., Zelaznik, H.N., Hawkins, B., Frank, J.S.,&Quinn, J.T. (1979). Motor-output variability: A theory for the accuracy of rapid motor acts. Psychological Review, 86, 415--451.
• 24: Shannon, C.E. (1948). A Mathematical Theory of Communication. Bell System Technical Journal, 27, 379--423 and 623--656.
• 25: R. William Soukoreff , I. Scott MacKenzie, Towards a standard for pointing device evaluation, perspectives on 27 years of Fitts' law research in HCI, International Journal of Human-Computer Studies, v.61 n.6, p.751-789, December 2004 [doi>10.1016/j.ijhcs.2004.09.001]
• 26: Thompson, D'Arcy W. (1917 and later editions). On Growth and Form. Cambridge (UK): Cambridge University Press.
• 27: Welford, A.T. (1968). Fundamentals of skill. London: Methuen.
• 28: Woodworth, R.S. (1899). The accuracy of voluntary movement. Psychological Review, 3, (Monograph Supplement), 1--119.
• 29: Wright, C.E. (1993). Evaluating the special role of time in the control of handwriting. Acta Psychologica, 82, 5--52.
• 30: Shumin Zhai, Characterizing computer input with Fitts' law parameters: the information and non-information aspects of pointing, International Journal of Human-Computer Studies, v.61 n.6, p.791-809, December 2004 [doi>10.1016/j.ijhcs.2004.09.006]
• 31: Shumin Zhai , Michael Hunter , Barton A. Smith, The metropolis keyboard - an exploration of quantitative techniques for virtual keyboard design, Proceedings of the 13th annual ACM symposium on User interface software and technology, p.119-128, November 06-08, 2000, San Diego, California, United States [doi>10.1145/354401.354424]

INDEX TERMS

Primary Classification:
  H. Information Systems
  H.5 INFORMATION INTERFACES AND PRESENTATION (I.7)
      H.5.2 User Interfaces (D.2.2, H.1.2, I.3.6)
          Subjects: Theory and methods

Additional Classification:
  H. Information Systems
  H.5 INFORMATION INTERFACES AND PRESENTATION (I.7)
      H.5.2 User Interfaces (D.2.2, H.1.2, I.3.6)
          Subjects: Evaluation/methodology

General Terms:
Experimentation, Human Factors

Keywords:
Fitts' law, experimental design, methodology, movement amplitude, movement difficulty, movement form, movement scale, movement shape, target distance, target tolerance, target width, theory