A formula is derived for the asymptotic standard error of a true-score equating by item response theory. The equating method is applicable when th two tests to be equated are administered to different groups along with an anchor test. Numerical standard errors are shown for an actual equating (1) comparing the standard errors of IRT, linear, e and equipercentile methods and (2) illustrating the effect of the length of the anchor test on the standard error of the equating.
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