A shift in task routines during the learning of a motor skill: group-averaged data may mask critical phases in the individuals' acquisition of skilled performance.

The authors describe a transient phase during training on a movement sequence wherein, after an initial improvement in speed and decrease in variability, individual participants' performance showed a significant increase in variability without change in mean performance speed. Subsequent to this phase, as practice continued, variability again decreased, performance significantly exceeded the gains predicted by extrapolation of the initial learning curve, the type of errors committed changed, and performance became more coherent. The transient phase of increased variability may reflect a mixture of 2 (or more) performance routines before the more effective one is set and mastered, presumably the setting up of a sequence-specific representation. Both group and individual analyses indicated a departure from the single process (e.g., power-law) model of learning. However, although similar phases appeared in the mean group data, there was little correspondence to individual participants' time courses, and the individuals' gains in the second low-variability phase were masked.