This book's purpose is to enable educational researchers to make better use of the huge longitudinal and cross-sectional data files that are now readily available. Of value to experienced researchers and undergraduates alike, the book focuses primarily on the planning and design of research rather than implementation of data base information. Its chapters address a variety of vital questions including: * What variables are present in each of the major national data bases? * What are the problems and dangers in making comparisons across data bases? * What factors invalidate comparisons of test scores across data bases? * In what ways can data from certain data bases be physically merged with data from other data bases? This text clearly demonstrates what can be done with large national data bases and, perhaps equally important, what cannot be done -- or done only if certain precautions are taken.