In an experiment on the effects of massed versus distributed practice on learning, introductory psychology students who volunteered for extra credit were randomly assigned to one of two groups. Both groups were to practice a perceptual-motor task five times for 10 minutes each. Group 1 did all five 10-minute practice sessions in one evening while group 2 was asked to do one 10-minute practice session a night for five consecutive nights. All of the participants in Group 1 completed the experiment, but only 30% of the participants in Group 2 showed up for all five nights. This study is an example of confounding by .