Critical Index of Difficulty for Different Body Motions: A Review

Abstract

K.-C. Gan and E. R. Hoffmann (1988) demonstrated that there is a clear boundary between values of Fitts' index of difficulty (ID) below which movements may be made without ongoing path corrections; above this value it was necessary to use corrections to hit a target of high ID. It is shown that this critical ID (IDcrit) value is dependent on both the body component making the movement and on the form of feedback available to the person. The arm system, having the highest ratio of muscle strength to mass moment of inertia, has the highest value of IDcrit, whereas control of the body center of mass location has the lowest value of IDcrit. It is suggested that the use of visual control to lower ID values with the larger body components is due to the longer movement times available because of the physical characteristics of these components-the time is available, and therefore it is used to make path corrections.

Rethinking the Dual Gaussian Distribution Model for Predicting Touch Accuracy in On-screen-start Pointing Tasks

“…Previous studies reported that the does not strongly influence the endpoint distribution [8,25,63]. For these typical pointing tasks, participants perform closed-loop motions [20]. In contrast, the endpoint distribution for when participants perform a ballistic motion has been reported to be affected by [20,38,46,61].…”

“…For these typical pointing tasks, participants perform closed-loop motions [20]. In contrast, the endpoint distribution for when participants perform a ballistic motion has been reported to be affected by [20,38,46,61]. Beggs et al [6,7] formulated the relationship in this way:…”

“…The critical threshold of whether participants perform a closed-loop or ballistic motion depends on Fitts' original index of difficulty, = log 2 (2 / ). When is less than 3 or 4 bits, a pointing task can be accomplished with only a ballistic motion [17,20]. While the critical changes depending on the experimental conditions, an extremely easy task with a short or large generally does not require precise closed-loop operations.…”

“…Nevertheless, the changes in obs and the success rate due to might be small in practice. This is because extremely short values, where users can accomplish a pointing task with only a single ballistic movement [17,20], also reduce the impact on obs according to Equation 11. If so, then, from a practical viewpoint, it would not be problematic to apply Bi and Zhai's success-rate prediction model to pointing from an on-screen start; therefore, we empirically validated this.…”

“…This study was a 4 × 5 within-subjects design. We included four s (20,30,45, and 60 mm) and five s (2, 4, 6, 8, and 10 mm). This covered a wide range of IDs requiring ballistic to closed-loop motions.…”