Using nationally representative data generated by the normal, day-to-day operation of schools, this research tested the major conclusions drawn from case studies focusing on principal effectiveness. On the basis of detailed information regarding individual student characteristics and achievements, detailed characteristics of teachers and their instructional choices and methods, and detailed information on the characteristics and behavior of principals, relevant changes in the educational process and in student outcomes were measured in relation to particular types of principal behavior. After the introduction, section 2 reviews the literature on the principal's role in student achievement. In section 3, a general contextual model for student achievement is presented, and the research methodology is described in detail, including the educational production function approach to the model and the specification of the variables for principal behavior and attributes. The empirical results are presented in section 4. The study concludes that principals directly affect student achievement through providing strong leadership and reducing conflicts among the participants in the educational process. References are included, along with an appendix listing variables used in the analysis. (TE)