Disentangling relative from absolute amplitude in Fitts' law experiments

ABSTRACT

Target distance (D) and target width (W), traditionally treated as independent variables in Fitts' target acquisition paradigm, are shown to suffer inextricable confounds with task difficulty. Through a simple geometrical analogy, it is shown that relative movement amplitude D/W(which determines difficulty) and absolute movement amplitude D (or scale) are the only two variables that can be manipulated independently in a Fitts' task experiment. Disentangling relative amplitude from absolute amplitude with an appropriate experimental design should help to study target acquisition in graphical user interfaces.

REFERENCES

• 1: Card, S. K., English, W. K., and Burr, B. J. Evaluation of mouse, rate-controlled isometric joystick, step keys, and text keys for text selection on a CRT. Ergonomics, 21, 8 (1978), 601-613.
• 2: Sarah A. Douglas , Arthur E. Kirkpatrick , I. Scott MacKenzie, Testing pointing device performance and user assessment with the ISO 9241, Part 9 standard, Proceedings of the SIGCHI conference on Human factors in computing systems: the CHI is the limit, p.215-222, May 15-20, 1999, Pittsburgh, Pennsylvania, United States [doi>10.1145/302979.303042]
• 3: Fitts, P. M. The information capacity of the human motor system in controlling the amplitude of movement. J. Exp. Psychol. 47, 6 (1954), 381-391.
• 4: Yves Guiard , Michel Beaudouin-Lafon , Denis Mottet, Navigation as multiscale pointing: extending Fitts' model to very high precision tasks, Proceedings of the SIGCHI conference on Human factors in computing systems: the CHI is the limit, p.450-457, May 15-20, 1999, Pittsburgh, Pennsylvania, United States [doi>10.1145/302979.303128]
• 5: Jagacinski, R. J. & Monk, D. L. (1985). Fitts' law in two dimensions with hand and head movements. J. Motor Behav. 17, 1 (1985), 77-95.
• 6: Gan, K. C., & Hoffmann, E. R. Geometrical conditions for ballistic and visually controlled movements. Ergonomics, 31, (1988), 829-839.
• 7: I. Scott MacKenzie, Movement time prediction in human-computer interfaces, Proceedings of the conference on Graphics interface '92, p.140-150, September 1992, Vancouver, British Columbia, Canada
• 8: Meyer, D. E., Smith, J. E. K., Kornblum, S., .Abrams, R. A., and Wright, C. E. Optimality in human motor performance. Psychol. Rev. 95,3 (1988), 340-370.
• 9: Schmidt, R. A., Zelaznik, H. N., Hawkins, B., Frank, J. S., and Quinn, J. T. Jr. Motor-output variability: A theory for the accuracy of rapid motor acts. Psychol Rev. 86, 5 (1979), 415-451.
• 10: Welford, A. T., Norris, A. H., & Shock, N. W. Speed and accuracy of movements and their changes with age. Acta Psychol. 30 (1969), 3-15.

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.1 MODELS AND PRINCIPLES
      H.1.2 User/Machine Systems
          Subjects: Human factors
  H.5 INFORMATION INTERFACES AND PRESENTATION (I.7)
      H.5.2 User Interfaces (D.2.2, H.1.2, I.3.6)
          Subjects: Graphical user interfaces (GUI)

General Terms:
Algorithms, Design, Human Factors

Keywords:
Fitts' law, amplitude, difficulty, input, pointing, scale