This is the first practitioner-oriented guide to basic psychometrics-the science behind test construction and validation. It is an essential course text and clinical reference for anyone who needs to select and administer diagnostic tests or review test score data obtained by others. Without assuming any mathematical background, the author describes the fundamentals of diagnostic testing, how to interpret score data, and how specific tests are used in school psychology, special education, counseling, and other contexts. Concepts such as reliability, validity, and fairness and test bias are explained in an accessible, conversational way. Detailed case examples illustrate the dos and don'ts of presenting test score data to clients, parents, and teachers in oral feedback and written reports. End-of-chapter exercises with answers emphasize application and understanding.