The Perils of Confounding Factors

Abstract

The design of Fitts' historical reciprocal tapping experiment gravely confounds index of difficulty ID with target distance D: Summary statistics for the candidate Fitts model and a competing model may appear identical, and the validity of Fitts' model for some tasks can be legitimately questioned. We show that the contamination of ID by either target distance D or width W is due to the common practices of pooling and averaging data belonging to different distance-width (D,W) pairs for the same ID, and taking a geometric progression for values of D and W. We analyze a case study of the validation of Fitts' law in eye-gaze movements, where an unfortunate experimental design has misled researchers into believing that eye-gaze movements are not ballistic. We then provide simple guidelines to prevent confounds: Practitioners should carefully design the experimental conditions of (D,W), fully distinguish data acquired for different conditions, and put less emphasis on r 2 scores. We also recommend investigating the use of stochastic sampling for D and W.

The Challenges in Modeling Human Performance in 3D Space with Fitts’ Law

“…Generally, an extension is deemed to be "superior" when tested against other models when there is a better correlation. However, there are numerous disadvantages to merely using the (R 2 ), which is supported by current literature [17,26].…”

Comparing Gaze, Head and Controller Selection of Dynamically Revealed Targets in Head-Mounted Displays

“…In practice, studies have routinely found gaze to perform in accordance with Fitts' Law [26,41]. Researchers have suggested that these results are due to secondary corrective saccades when targets are small [51], or artefacts of experimental conditions or analysis methodology [21]. For this work, it also has to be noted that gaze is not always performed by the eyes alone.…”

“…Whether gaze pointing is appropriately modelled by Fitts Law has become a matter of considerable debate [11,21,51]. Eye movement, unlike head and hand movement, is ballistic in nature.…”

“…In this work, we evaluate the applicability of these models also for head and gaze pointing. Specifically for gaze, it has been argued that movement time should not depend on target width as gaze saccades are pre-programmed by the visual system as a ballistic movement [11,21]. We have therefore also considered Carpenter's model of saccade duration, solely depending on target distance, for the prediction of gaze performance [9].…”