This report focuses on three main questions: (1) To what extent does a Head Start experience accelerate the rate at which disadvantaged preschoolers acquire cognitive skills? (2) Are the Planned Variation models, simply by virtue of sponsorship more effective than ordinary nonsponsored Head Start programs? and (3) Are some Planned Variation models particularly effective at imparting certain skills? The first chapter gives an overall picture of the Head Start Planned Variation study, while the second chapter summarizes data concerning background characteristics and distribution of test scores. Chapter 3 provides a general discussion of methodological issues and some of the major difficulties resulting from the study design. Chapters 4-7 attempt to present a picture of the pattern of overall effects of various programs through ranking analysis, residual analysis, analysis of covariance, and resistant analysis. The final chapters explore the question of whether the relative effectiveness of various programs is related to certain child background characteristics, such as sex, ethnicity, age, prior school experience, and mother's education. One major conclusion drawn as a result of the intermodel comparisons was that Head Start programs are quite homogeneous in their ability to promote general cognitive development. (CS)