A comparison of three polytomous item response theory models in the context of testlet scoring.

An alternative to dichotomous scoring of multiple items anchored to a common stem is scoring these items as a single polytomous item (testlet scoring). This study systematically compared the partial credit model (PCM), the generalized partial credit model (GPCM), and the graded response model (GRM) in the context of testlet scoring. Data sets included a sample from the fall 1994 administration of the SAT I (N = 2,548) and a simulated data set. Theta estimation, information, and model fit were analyzed. Correlations among theta estimates ranged from 0.9748 to 0.9921. The relationship among the information functions of the PCM, GPCM and the GRM reflected the discrimination parameter estimates for the latter two models. Suggestions are made with regard to model selection.