Inferences about time course of Weber’s Law violate statistical principles

Recently, Holmes et al. (2011b) suggested that grasping is only subject to Weber’s Law at early but not late points of a grasping movement. They therefore conclude that distinct visual computations and information may guide early and late portions of grasping. Here, we argue that their results can be explained by an interesting statistical artifact, and cannot be considered indicative of the presence or absence of Weber’s Law during early portions of grasping. Our argument has implications for other studies using similar methodology (e.g., Heath et al., 2011, Holmes et al., 2011a and Holmes et al., 2012), and also for the analysis of temporal data (often called time series) in general.

► Temporal noise causes standard deviation (SD) to be dependent on function slope.
► Grip aperture SDs depend on object size because profile slope does.
► The method of averaging trajectories is not informative of Weber’s Law.