Comparisons and Contrasts Between Item Response Theory and Rasch Measurement Theory

There are two approaches that can be taken with regards to a data–model relationship. The first and most common approach is that of item response theory (IRT), where the model describes the data. If the data do not fit the model, another model with more parameters is chosen. The second and less common approach is that of Rasch measurement theory, where the model is an operationalization of fundamental requirements of measurement. If the data do not fit the model, reasons for the misfit are investigated as anomalies.