Comparison of Dichotomous and Polytomous Item Response Models in Equating Scores from Tests Composed of Testlets

The performance of two polytomous item response theory models was compared to that of the dichotomous three-parameter logistic model in the context of equating tests composed of testlets. For the polytomous models, testlet scores were used to eliminate the effect of the dependence among within-testlet items. Traditional equating methods were used as criteria for both. The equating methods based on polytomous models were found to produce results that more closely agreed with the results of traditional methods.