The first practitioner-oriented guide to basic psychometrics, this is an essential course text and clinical reference for anyone who needs to 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, counseling, and other contexts. Reliability, validity, and fairness and test bias are explained in an accessible way; case examples illustrate how to present test score data in oral feedback and written reports. Pedagogical features include end-of-chapter exercises with answers.