The measurement theories considered in this book fall into a sub-area of psychometrics. These theories are concerned with the specifications of rules for transforming individuals' responses to items on a psychological test or scale into estimates of the trait assumed to underlie the observable responses. Discussed is a particular class of measurement theories generally called item response theory or latent trait theory. This book is designed for those individuals involved in programs of applied testing and measurement whose efforts occasionally bring them into contact with the law and public policy.