Attrition: Identification and Exploration in the National Follow Through Evaluation.

A general interest in attrition, or loss of units from a study, stems in part from the observation that the infrequency of attention to attrition exacerbates problems of data interpretation. As a substudy of the national evaluation of Project Follow Through, the potential biasing effects of attrition of subjects from the sites were investigated. Policy attrition, the administrative dropping of a unit; program attrition, loss of subjects due to mobility, dislike of the treatment, etc.; and sample attrition, units omitted because of deficient data, have been defined, but were not distinguished in this research. The relationship between attrition, pretest score, and income level were studied. In the few cases where additional variance was accounted for by the interaction of pretest or income with treatment within-site, an interpretation of the possible biasing effects of attrition was made in the evaluation of the specific site. It is suggested that in cases where differential attrition is found, the offending variable be considered for use as a covariate. (BW)