The Third Edition of the Encyclopedia of Educational Research marks another milestone in the progress of research in education. It reflects the upsurge of educational research during the past decade. It conveys the vision and vitality of a new generation of workers in research. Yet it retains the essential values so well established in the first and revised editions. Educational research is moving at an ever-accelerating rate. Keeping the Encyclopedia abreast of the rapidly expanding volume of research in education presents a continuing challenge. The Third Edition was conceived as a new edition that would serve the established purpose of presenting for each topic a critical evaluation, synthesis, and interpretation of all the pertinent research-early as well as recent. It reflects both a tradition ably established by Walter S. Monroe and the many contributors to the first two editions, and a desire to reexamine our conceptions of educational research and the generalizations to which such studies have led. In planning this edition a new analysis of the articles to be included was made, and each article was developed as a new and complete treatment of the topic that would consider all the relevant research regardless of the period during which it had appeared. This edition was planned not simply as a revision of earlier editions, but as a completely rewritten volume that would attempt to put into a new perspective the findings of research. Consistent with the decisions to adopt a somewhat different organization and to ask for more than a revision of the existing articles, many new contributors have participated. Every contributor is an outstanding leader in American education.