All 23 chapters of the book have been revised following the suggestions of numerous reviewers. An attempt was made to address reviewers' concerns, to update material where needed, add missing perspectives, correct errors, and generally freshen and streamline the treatment. Suggested readings have been shortened and updated, drawing mainly from literature published since the publication of the third edition.

The presentation has been reformatted to help students orient to the flow of information in each chapter. The concepts in each chapter are now numbered both in the first listing of chapter concepts and at the beginning of each section in which concepts are discussed. The concept numbers are repeated in the concept review questions that conclude each concept section. Thus the beginning and end of all concepts are signaled clearly for the student. Each "Investigating the Evidence" boxed discussion is also given the number of the chapter in which it appears, again as a locator for the student.

Over 240 study questions have been added throughout the text to help students review the major concepts. The "Concept Review" questions that conclude each concept discussion are designed to help students think critically about content and to encourage them to reflect on the design of research projects and on the thinking of researchers. "Investigating the Evidence" boxed discussions include questions called "Critiquing the Evidence," which are intended to explore some of the details of the statistics and study design topics presented.

Two new appendixes provide the answers to the more than 240 "Concept Review" and "Critiquing the Evidence" questions.

A list of key terms at the end of each chapter alerts students to new vocabulary as it is introduced.