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NEW TO THIS EDITION: • New topics expose students to current research issues in human development. Topics include: emotional development, bullying, early forms of moral understanding, the “replication crisis” in psychology, the role of gestures in cognitive development, the study of false belief in infancy, the “teenage brain” and its implications for adolescent behavior, the study of the “oldest old,” and the population of centenarians. • Key Terms lists now appear at the end of each chapter to help students master the vocabulary of research methods. • New boxes, exercises, glossary items, and tables and figures bring the book completely up to date. • Approximately 400 new references reflect recent scholarship in the field KEY FEATURES: • Flexible chapters provide instructors the ability to cover material in any order they prefer. • A student-friendly design and engaging approach provides extended coverage of especially interesting and important contemporary topics through chapter boxes, tables, figures, and photos. • Built-in study tools, including exercises, chapter-ending summaries, key terms lists, a glossary, citation of further sources, and relevant websites, help students master key content.

Design

Chapter 3 design

We saw in Chapter 2 that all research involves comparison. In most cases, the comparison is between different levels of an independent variable. If the independent variable is a nonmanipulable subject characteristic such as age, then the researcher must select participants who already possess different levels of the characteristic. If the independent variable is an experimentally manipulable factor, then the researcher must assign participants to conditions that embody the desired levels of the factor. In either case, the researcher must do the selecting and assigning in a way that will allow a clear, nonconfounded comparison of the different levels being studied (the internal validity question), that will permit generalization to other samples and situations of interest (the external validity question), and ...

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