Relationships Between the Thurstone and Rasch Approaches to Item Scaling

When the logistic function is substituted for the normal, Thurstone's Case V specialization of the law of comparative judgment for paired comparison responses gives an identical equation for the esti mation of item scale values as does the Rasch formulation for direct responses. The law of com parative judgment must be modified to include a subject parameter; but this parameter, which is eliminated statistically with respect to the direct re sponse design, is eliminated experimentally in the paired comparison design. Some comparisons and contrasts are made between the two approaches to item scaling, and it is shown that greater gen eralizability for item scaling is possible when the two approaches are juxtaposed appropriately.