In Schmidt's experiments, only properties of actually produced movements systems are measured; in Fitts' experiments, external task parameters are measured too (target size and distance). Thus, the laws contain variables of different natures and cannot be reduced to each other even formally. These difference especially reveals itself in modeling: a model of variability can be simpler if it deals with the performance variables only. On the other hand, modeling Fitts' law, one should take into account not only the human effector system but features of the experimental apparatus as well. These differences have not been sufficiently reflected in Plamondon & Alimi's target article.