Comparison of IRT True-Score and

Two methods of ’equating ’ tests are compared, one using true scores, the other using equipercentile equat-ing of observed scores. The theory of equating is dis-cussed. For the data studied, the two methods yield almost indistinguishable results. Most item response theory (IRT) equating is currently attempted by the true-score equating procedure described in Lord (1980, chap. 13). Lord also described an IRT equipercentile observed-score procedure, which until now seems to have been little used in operational work, perhaps because it is more complicated and more expensive than the true-score procedure. This paper discusses theoretical considerations and reports an empirical research study comparing the results of applying these two procedures to real test data. Note that IRT plays only a subsidiary role in observed-score equipercentile equating; similar results could be expected for conventional equipercentile equating, assuming that the IRT model holds. Kolen (1981) found the equipercentile observed-score IRT procedure to be one of the better of nine procedures compared in his empirical research study. However, his criterion was stability in cross-validation. Although stability is certainly desirable, stability is not a proper criterion for choosing th