Fitts's law

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For the use of the word "Fitt" in Old English poetry, see Beowulf.

Fitts's law (often cited as Fitts' law) is a descriptive model of human movement primarily used in human–computer interaction and ergonomics that predicts that the time required to rapidly move to a target area is a function of the ratio between the distance to the target and the width of the target.[1] Fitts's law is used to model the act of pointing, either by physically touching an object with a hand or finger, or virtually, by pointing to an object on a computer monitor using a pointing device.

Fitts's law has been shown to apply under a variety of conditions, with many different limbs (hands, feet,[2] head-mounted sights,[3] eye gaze), manipulanda (input devices), physical environments (including underwater), and user populations (young, old, special educational needs, and drugged participants).

Original model formulation[edit]

The original formulation of Fitts's law, proposed by Paul Fitts in 1954, is based on the intuition that movement time is the sum of a constant reaction or sub-task time (e.g. clicking a mouse, tapping a stylus) and a variable movement time linearly dependent on quantified task difficulty:

T = a + b(ID) = a + b \log_2 \Bigg(\frac{2D}{W}\Bigg)

where:

  • T is the average time taken to complete the movement.
  • a and b are model parameters.
  • ID is the Index of Difficulty term; a number of variants on the definition of this term exist.
  • D is the distance from the starting point to the center of the target.
  • W is the width of the target measured along the axis of motion. W can also be thought of as the allowed error tolerance in the final position, since the final point of the motion must fall within ±W2 of the target's center.

Since shorter movement times are desirable for a given task, the value of the b parameter can be used as a metric when comparing computer pointing devices against one another. The first Human-Computer Interface application of Fitts's law was by Card, English, and Burr (1978), who used the index of performance (IP), defined as 1b, to compare performance of different input devices, with the mouse coming out on top compared to the joystick or directional movement keys.[4] This early work, according to Stuart Card's biography, "was a major factor leading to the mouse's commercial introduction by Xerox".[5]

Many experiments testing Fitts's law apply the model to a dataset in which either distance or width, but not both, are varied. The model's predictive power deteriorates when both are varied over a significant range.[6] Notice that because the ID term depends only on the ratio of distance to width, the model implies that a target distance and width combination can be re-scaled arbitrarily without affecting movement time, which is impossible. Despite its flaws, this form of the model does possess remarkable predictive power across a range of computer interface modalities and motor tasks, and has provided many insights into user interface design principles.

Bits per second: model innovations driven by information theory[edit]

The formulation of Fitts's law most frequently used in the Human-Computer Interaction community is called the Shannon form, and varies in the definition of the ID term:

T = a + b \log_2 \Bigg(1+\frac{D}{W}\Bigg)

This form was proposed by Scott MacKenzie, professor at York University, and named for its resemblance to the Shannon–Hartley theorem.[7]

Using this form of the model, the difficulty of a pointing task was equated to a quantity of information transmitted (in units of bits) by performing the task. This was justified by the assertion that pointing reduces to an information processing task. Although no formal mathematical connection was established between Fitts's law and the Shannon-Hartley theorem it was inspired by, the Shannon form of the law has been used extensively, likely due to the appeal of quantifying motor actions using information theory. In 2002 the ISO 9241 was published, providing standards for human-computer interface testing, including the use of the Shannon form of Fitts's law. It has been shown that the information transmitted via serial keystrokes on a keyboard and the information implied by the ID for such a task are not consistent.[8]

Welford's model: innovations driven by predictive power[edit]

Not long after the original model was proposed, a 2-factor variation was proposed under the intuition that target distance and width have separate effects on movement time. Welford's model, proposed in 1968, separated the influence of target distance and width into separate terms, and provided improved predictive power:[9]

T = a + b_1 \log_2 (D) + b_2 \log_2 (W)

This model has an additional parameter, so its predictive accuracy cannot be directly compared with 1-factor forms of Fitts's law. However, a variation on Welford's model inspired by the Shannon formulation,

T = a + b_1 \log_2 (D+W) + b_2 \log_2 (W) = a + b\log_2 \left(\frac{D+W}{W^k}\right)

reduces to the Shannon form when k = 1. Therefore, this model can be directly compared against the Shannon form of Fitts's law using the F-test of nested models.[10] This comparison reveals that not only does the Shannon form of Welford's model better predict movement times, but it is also more robust when control-display gain (the ratio between e.g. hand movement and cursor movement) is varied. Consequently, although this model is slightly more complex and less intuitive, it is empirically the best model to use for virtual pointing tasks.

Extending the model from 1D to 2D and other nuances[edit]

Extensions to two or more dimensions[edit]

In its original form, Fitts's law is meant to apply only to one-dimensional tasks. However, the original experiments required subjects to move a stylus (in three dimensions) between two metal plates on a table, termed the reciprocal tapping task. The target width perpendicular to the direction of movement was very wide to avoid it having a significant influence on performance. A major application for Fitts's law is 2D virtual pointing tasks on computer screens, in which targets have bounded sizes in both dimensions.

Fitts's law has been extended to two-dimensional tasks in two different ways. For navigating e.g. hierarchical pull-down menus, the user must generate a trajectory with the pointing device that is constrained by the menu geometry; for this application the Accot-Zhai steering law was derived.

For simply pointing to targets in a two-dimensional space, the model generally holds as-is but requires adjustments to capture target geometry and quantify targeting errors in a logically consistent way.[11] The concept of "effective width" was developed to reduce a potentially complex 2D target shape to a single number for use in Fitts's law.[7]

Characterizing performance[edit]

Since the a and b parameters should capture movement times over a potentially wide range of task geometries, they can serve as a performance metric for a given interface. In doing so, it is necessary to separate variation between users from variation between interfaces. The a parameter is typically positive and close to zero, and sometimes ignored in characterizing average performance. Multiple methods exist for identifying parameters from experimental data, and the choice of method is the subject of heated debate, since method variation can result in parameter differences that overwhelm underlying performance differences.[12][13]

An additional issue in characterizing performance is incorporating success rate: an aggressive user can achieve shorter movement times at the cost of experimental trials in which the target is missed. If the latter are not incorporated into the model, then average movement times can be artificially decreased. "Crossman's correction" addresses this problem by defining target width post-hoc as the size required to capture 96% of trial endpoints (e.g. click locations on a computer screen).[14]

See also[edit]

Notes[edit]

  1. ^ Fitts, Paul M. (June 1954). "The information capacity of the human motor system in controlling the amplitude of movement". Journal of Experimental Psychology 47 (6): 381–391. doi:10.1037/h0055392. PMID 13174710. 
  2. ^ Hoffmann, Errol R. (1991). "A comparison of hand and foot movement times". Ergonomics 34 (4): 397–406. doi:10.1080/00140139108967324. 
  3. ^ So, R. H. Y.; Griffin, M. J. (2000). "Effects of target movement direction cue on head-tracking performance". Ergonomics 43 (3): 360–376. doi:10.1080/001401300184468. 
  4. ^ Card, Stuart K.; English, William K.; Burr, Betty J. (1978). "Evaluation of mouse, rate-controlled isometric joystick, step keys, and text keys for text selection on a CRT". Ergonomics 21 (8): 601–613. doi:10.1080/00140137808931762. 
  5. ^ "Stuart Card". PARC. Archived from the original on 2012-07-11. 
  6. ^ Graham, Evan (1996). Pointing on a Computer Display (Ph.D.). Simon Fraser University. 
  7. ^ a b MacKenzie, I. Scott (1992). "Fitts' law as a research and design tool in human–computer interaction" (pdf). Human–Computer Interaction 7: 91–139. doi:10.1207/s15327051hci0701_3. 
  8. ^ Soukoreff, R. William; Zhao, Jian; Ren, Xiangshi (2011). "The Entropy of a Rapid Aimed Movement: Fitts' Index of Difficulty versus Shannon's Entropy". Human Computer Interaction: 222–239. 
  9. ^ Welford, A. T. (1968). Fundamentals of Skill. Methuen. 
  10. ^ Shoemaker, Garth; Tsukitani, Takayuki; Kitamura, Yoshifumi; Booth, Kellogg (December 2012). "Two-Part Models Capture the Impact of Gain on Pointing Performance". ACM Transactions on Computer-Human Interaction. 19(4):1-34. 
  11. ^ Wobbrock, J.; Shinohara, K (2011). "The effects of task dimensionality, endpoint deviation, throughput calculation, and experiment design on pointing measures and models.". Proceedings of the ACM Conference on Human Factors in Computing Systems (Vancouver, British Columbia): 1639–1648. 
  12. ^ Soukoreff, R. William; MacKenzie, I. Scott (2004). "Towards a standard for pointing device evaluation, perspectives on 27 years of Fitts' law research in HCI". International Journal of Human-Computer Studies 61 (6): 751–789. doi:10.1016/j.ijhcs.2004.09.001. 
  13. ^ Zhai, Shumin (2002). On the Validity of Throughput as a Characteristic of Computer Input (pdf) (Technical report). San Jose, California: Almaden Research Center. RJ 10253. 
  14. ^ MacKenzie, I. Scott; Buxton, William A. S. (1992). "Extending Fitts' law to two-dimensional tasks". Proceedings of ACM CHI 1992 Conference on Human Factors in Computing Systems: 219–226. doi:10.1145/142750.142794. (subscription required (help)). 

References[edit]

  • Accot, Johnny; Zhai, Shumin (2002). "More than dotting the i's—foundations for crossing-based interfaces". Proceedings of ACM CHI 2002 Conference on Human Factors in Computing Systems: 73–80. doi:10.1145/503376.503390. (subscription required (help)). 
  • Accot, Johnny; Zhai, Shumin (2003). "Refining Fitts' law models for bivariate pointing". Proceedings of ACM CHI 2003 Conference on Human Factors in Computing Systems: 193–200. doi:10.1145/642611.642646. (subscription required (help)). 
  • Card, Stuart K.; Moran, Thomas P.; Newell, Allen (1983). The Psychology of Human–Computer Interaction. Hillsdale, NJ: L. Erlbaum Associates. ISBN 0898592437. 
  • Fitts, Paul M.; Peterson, James R. (February 1964). "Information capacity of discrete motor responses". Journal of Experimental Psychology 67 (2): 103–112. doi:10.1037/h0045689. 

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