APPLICATION OF A GENERAL POLYTOMOUS TESTLET MODEL TO THE READING SECTION OF A LARGE‐SCALE ENGLISH LANGUAGE ASSESSMENT

Many standardized educational tests include groups of items based on a common stimulus, known as testlets . Standard unidimensional item response theory (IRT) models are commonly used to model examinees' responses to testlet items. However, it is known that local dependence among testlet items can lead to biased item parameter estimates when using standard IRT models, and to overestimated reliability. In this study, a general polytomous testlet model was proposed to account for local dependence in testlet‐based tests that contain both dichotomously and polytomously scored items. The proposed model and a standard IRT model were fit to simulated data and several real data sets from the reading sections of a large‐scale English‐language test, and model fit was evaluated. Item parameters and test information obtained from the two models were compared to check the impact of local item dependence. In addition, a multidimensional IRT model with simple structure was also fit to the real data sets. Results based on both simulated and real data suggested that local dependence had a small impact on item parameter estimates and a relatively larger impact on test information and reliability. It was also found that the multidimensional IRT model with simple structure fit the real data sets better than the general polytomous testlet model and the standard IRT model did.