Incorrect handling of item parameter drift during the equating process can result in equating error. If the item parameter drift is due to construct-irrelevant factors, then inclusion of these items in the estimation of the equating constants can be expected to result in equating error. On the other hand, if the item parameter drift is related to the construct being measured, then removal of those items from the estimation of the equating constants can be expected to result in equating error. The effect of incorrect handling of item parameter drift on equating has been examined via simulation studies and other methods by a number of authors with mixed results. In this article, this effect is assessed by estimating the expected equating error under the assumptions that the three-parameter item response theory model is used for calibration-equating. (Contains 1 table.)