An Empirical Study of Vertical Equating Methods Using the Three-Parameter Logistic Model.

The accuracy of trait estimates obtained from three vertical equating methods was examined. The procedures studied included two anchor test designs and a single-group design. Data from two content areas and two grade combinations were studied. A three-parameter logistic model was used to perform the equatings. The results obtained were used to produce pairs of true score estimates for a cross-validation sample, one estimate for each of the equated tests. Comparisons among pairs of estimates indicated that the single-group method consistently produced the most accurate results, although the advantage was small. A differential effect due to content area was reported for the two anchor test methods. (Author)