Psychometrics is the field of study concerned with the theory and technique of educational measurement and psychological measurement, which includes the measurement of knowledge, abilities, attitudes, and personality traits. The field is primarily concerned with the construction and validation of measurement instruments, such as questionnaires, tests, and personality assessments. Since the founding of modern psychometrics, both the theory and practice have progressed vigorously. Tests are continuously developed, classical theories are extended and subsumed under modern theories, and new applications are designed. Unfortunately, test construction and test theory have developed separately. Test constructors are often badly informed about modern theories, and test theorists are badly informed about test construction. This book bridges the gap between theory and practice by discussing both test construction and test theory in psychometrics.