The Effects of Test Content Match and Number of Items on the Accuracy of Trait Estimates from Tests Equated with the Three-Parameter Logistic Model.

The purpose of the present study was to examine the accuracy of indirect trait estimates, i.e., estimates of some primary trait obtained from a second measure which have been equated to the first. The California Achievement Test in Reading was the primary measure and the Prescriptive Reading Inventory was the indirect measure. Four kinds of content-match between primary and indirect item sets were studied: match, similar, dissimilar, and partial. Within each content-match category, indirect trait estimates based on item sets containing 10, 20, and 30 items were compared to those obtained from the primary measure. A three-parameter logistic model was applied to an internal anchor test design to equate the measures. The results obtained were then evaluated for a cross-validation sample. The accuracy of indirect trait estimates was affected by both content-match and the number of items used to produce the estimate. As indirect item sets became more similar in content or as the number of items increased, the accuracy of the estimates increased. However, a considerable amount of equating bias was noted, even for the 30-item sets. (Author/CM)