Irt Versus Conventional Equating Methods: A Comparative Study of Scale Stability

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Abstract
Scale drift for the verbal and mathematical portions of the Scholastic Aptitude Test (SAT) was investigated using linear, equipercentile and item response theory (IRT) equating methods. The linear methods investigated were the Tucker, Levine Equally Reliable and Levine Unequally Reliable models. Three IRT calibration designs were employed. These designs are referred to as (1) concurrent, (2) fixed b’s method, and (3) characteristic curve transformation method. The results of the various equating methods were compared both graphically and analytically. These results indicated that for reasonably parallel tests, linear equating methods perform adequately. However, when tests differ somewhat in content and length, methods based on the three-parameter logistic IRT model lead to greater stability of equating results. Of the conventional equating methods investigated, the Levine Equally Reliable model appears to be the most robust for the type of equating situation used in this study. The IRT method that provided the most stable equating results overall was the concurrent calibration method.