Developing skilled performance of lumbar spine manipulation

I read with interest the recent study by Triano et al1 comparing manipulative performance in 2 groups of chiropractic students. I am curious, however, as to how much of the learning effect observed in the experimental group to attribute to the use of the simulator. The experimental group practiced 3 procedures daily (1 cervical, 1 thoracic, 1 lumbar) with 10 repetitions, while the practice history of the control group was described as “ad lib.” Are the authors able to elaborate on the practice history of the control group, since the observed differences between groups may have been the result of the experimental group practicing more, regardless of simulator use?1 Second, motor skill performance often increases with increased variability of practice.2-3 Is it possible to attribute differences between groups to increased practice variability in the experimental group3?