This study investigated the impact of the coding scheme on IRT-based true score equating under a common-item nonequivalent groups design. Two different coding schemes under investigation were carried out by assigning either a zero or a blank to a missing item response in the equating data. The investigation involved a comparison study using actual large scale data and then Monte Carlo simulations for a systematic inspection on the topic. The recommendations on the basis of the findings of the study were made to treat omitted responses as not-administered rather than as wrong, and use a large sample size to ensure the accuracy of the screening tools such as the displacement index and the robust-"z" statistic during equating. (Contains 4 figures and 1 table.)