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A meta-analysis of substance abuse prevention studies revealed that the mean proportion of subjects retained dropped from 81.4% at 3-month to 67.5% at 3-year follow-ups. Time from pretest alone accounted for less than 5% of the variance. Other available predictors of retention were not significant. Researchers are encouraged to interpret their results in light of these normative data and to adopt second-effort strategies to reduce attrition.
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Hansen et al. (1990) studied this question.
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