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328 PART IV: Applied Researchassigned to conditions. Even when pretest scores show no difference betweengroups, however, we cannot assume that the groups are “equivalent” (Campbell& Stanley, 1966). We will explain why we cannot conclude that the groups areequivalent in the discussion that follows.Selection-Maturation Effect An additive effect of selection and maturation occurswhen individuals in one group grow more experienced, more tired, or morebored at a faster rate than individuals in another group (Shadish et al., 2002).A selection-maturation effect is more likely to be a threat to internal validitywhen the treatment group is self-selected (the members deliberately sought outexposure to the treatment) and when the comparison group is from a differentpopulation from the treatment group (Campbell & Stanley, 1966). Langerand Rodin selected their groups (but not individuals) randomly from the samepopulation of individuals. Consequently, their design more closely approachesa true experiment than it would if individuals in the two groups had come fromdifferent populations (Campbell & Stanley, 1966). A selection-maturation effectwould have been more likely, for example, if residents in a nursing home werecompared with those attending a sheltered workshop program for the elderly,or if residents on different floors of a nursing facility require different levelsof care.The possibility of a selection-maturation effect is one reason we cannotconclude the groups are equivalent (comparable) even when pretest scoresare the same on average for the treatment and control groups. The naturalgrowth rate of two groups from different populations might be different, butthe pretest may have been taken at a time when both groups happened to beabout the same. This problem is illustrated in Figure 10.4. The normal rateof change is greater in Group A than in Group B, but the pretest is likely toshow that the groups do not differ. Because of the differential growth rate,however, the groups would probably show a difference at the posttest thatcould be mistaken for a treatment effect. There is a second, and more general,FIGURE 10.4Possible differential growth rates for two groups (A and B) in the absence of treatment.Group ADependent VariableGroup BPretestTimePosttest