About five decades ago, the visionary Dutch psychologist A. D. De Groot started building an extraordinary academic group at the University of Amsterdam. It consisted of psychometricians, statisticians, philosophers of science, and psychologists with a general methodological orientation. The idea was to approach methodological problems in psychology from the various angles these different specialists brought to the subject matter. By triangulating their viewpoints, methodological problems were to be clarified, pinpointed, and solved. This idea is in several respects the basis for this book. At an intellectual level, the research reported here is carried out exactly along the lines De Groot envisaged, because it applies insights from psychology, philosophy of science, and psychometrics to the problem of psychological measurement. At a more practical level, I think that, if De Groot had not founded this group, the book now before you would not have existed. For the people in the psychological methods department both sparked my interests in psychometrics and philosophy of science, and provided me with the opportunity to start out on the research that is the basis for this book. Hence, I thank De Groot for his vision, and the people in the psychological methods group for creating such a great intellectual atmosphere.