A complex two-stage sample selection process was used in designing the National Longitudinal Study of the High School Class of 1972. The first-stage sampling frame used in the selection of schools was stratified by the following seven variables: public vs. private control, geographic region, grade 12 enrollment, proximity to institutions of higher education, percentage of minority group enrollment, community income level, and degree of urbanization. Six hundred strata determined the initial sample of 1,200 schools; later, a random sample of 18 seniors per high school was selected. This report considers the effects of stratification, oversampling of schools by percentage of minority group enrollment and community income level, clustering of students within a school and unequal weighting on the variances of the resulting statistics and therefore, on the precision of the sample statistics. Results suggest that school stratification variables reduced the variances of national estimates to twenty percent below what would have been expected with unstratified cluster sampling. Of the five major stratification variables: socioeconomic status, size of school, type of control, geographic region, and proximity to college or university; region is the strongest variable and type of control is the weakest. (Author/MV)