Using the dichotomous Rasch model to analyze polytomous items.

One of the most important applications of the Rasch measurement models in educational assessment is the equating of tests. An important feature of attainment tests is the use of both dichotomous and polytomous items. The partial credit model (PCM) developed by Masters (1982) represents an extension of the dichotomous Rasch model for analysing polytomous item data. The dichotomous Rasch model has been used primarily to analyse dichotomous item data. Whilst the partial credit model can provide detailed information on the performance of individual score categories of polytomous items, it is mathematically more complex to use than the dichotomous Rasch model and can, under certain circumstances, present difficulties in interpreting item measures and in practical applications. This study explores the potential of using the dichotomous Rasch model to analyse polytomous items and equate tests. Results obtained from a simulation study and from analysing the data of a science achievement test indicate that the partial credit model and the dichotomous Rasch model produce similar item and person measures and equivalent cut scores on different test forms.