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CHAPTER 6: Independent Groups Designs 199FIGURE 6.3Many people who begin a rigorous exercise program fail to complete it. In a sense, only the“fittest” survive, a situation that could cause problems of interpretation if different types of fitnessprograms were being compared.25 experimental participants who completed the program. The selective loss ofparticipants in the experimental group likely destroyed the comparable groupsthat were formed by random assignment at the beginning of the experiment. Infact, the final fitness scores of the 25 experimental participants might have beenhigher than the average in the control group even if they had not participated inthe fitness program because they were more fit when they began! Thus, the subjectloss in this experiment meets the other two conditions for selective subjectloss. Namely, the loss is likely due to a characteristic of the participants—theiroriginal level of fitness—and this characteristic is relevant to the outcome ofthe study (see Figure 6.3).If selective subject loss is not identified until after the experiment is completed,little can be done except to chalk up the experience of having conductedan uninterpretable experiment. Preventive steps can be taken, however, whenresearchers realize in advance that selective loss may be a problem. One alternativeis to administer a pretest and screen out subjects who are likely tobe lost. For example, in the exercise study, an initial test of fitness could havebeen given, and only those participants who scored above some minimal levelwould have participated in the experiment. Screening participants in this waywould involve a potential cost. The results of the study would likely apply onlyfor people above the minimal fitness level. This cost may be well worth payingbecause an interpretable study of limited generality is still preferable to an uninterpretablestudy.