The new rules of measurement

Classical test theory, which the authors maintain applied psychologists are still too often exclusively taught, are contrasted with the new rules of measurement. In the newer, model-based version of test theory, called item response theory (IRT), some well-known rules of measurement no longer apply. Six old rules bf measurement that conflict with the new rules are reviewed, and intuitive explanations of the new rules are provided. Readers are also directed to additional informational sources about IRT, which, it is argued, every psychologist should be familiar with.