This book develops an intuitive understanding of item response theory (IRT) principles through the use of graphical displays and analogies to psychological principles. The IRT psychometric method is especially useful to psychologists and other social scientists that are familiar with small scale cognitive and personality measures. Familiar concepts are used to explain various principles such as contemporary IRT models and estimation methods. Polytomous IRT models are given central coverage since many psychological tests use rating scales. Applications in various research areas are...