The Effect of Anchor Length and Equating Method on the Accuracy of Test Equating: Comparisons of Linear and IRT-Based Equating Using an Anchor-Item Design.

The influence of anchor length on the accuracy of test equating was studied using Tucker's linear method and two Item-Response-Theory (IRT) based methods, focusing on whether equating accuracy improved with more anchor items, whether the anchor effect depended on the equating method used, and the adequacy of the inclusion of the guessing parameter for a test that had a negatively skewed distribution of scores. Data were from 2 forms of a minimum competency examination that contained 197 and 203 items respectively. Three pairs of shorted forms were created by the simple random sampling of items, and the pairs were equated separately. The total score on the 145 anchor items was used as a criterion, a pseudo true score, to evaluate result accuracy. True score estimates were obtained that were correlated to the pseudo true score. Overall, results yielded by all three equating methods were moderately accurate, and no matter which equating method was used, the results tended to be more accurate when there were more anchor items. In addition, inclusion of a guessing parameter was justified. Six appendixes present the sampling scheme for the reduced forms, item correlations and descriptive test statistics for the reduced and full forms, and the two equating methods. (Contains 1 figure, 4 tables, 18 appendix tables, and 46 references.) (SLD)