Lack of random assignment. Random assignment of subjects into treatment and control groups is the ideal method of attributing change in a sample to an intervention and not to some other effect (including the pre-existing capabilities of students, teachers, or school systems). However, for a variety of practical reasons, this procedure was not done in "Follow Through". Instead, sites were selected "opportunistically", based on their readiness to participate in the evaluation, and on their unique circumstances of need. As Stebbins "et al." point out, the treatment groups were often the neediest children. To randomly select some of the most disadvantaged children (many of whom participated in Head Start prior to "Follow Through") out of the evaluation would certainly have been negatively perceived by community members. Stebbins "et al." point out that there were "considerable variations in the range of children served"; yet despite the presence of "many of the problems inherent in field social research...evaluations of these planned variations provides us with an opportunity to examine the educational strategies under real life conditions as opposed to contrived and tightly controlled laboratory conditions".