Previous work on the use of w for detection of answer copying was based on the assumption that item parameters for the nominal response model were known. Such an assumption limits the usefulness of a, particularly in the classroom, because most teachers do not have a set of precalibrated items. This study investigated empirical Type I error rates and the power of w when the item and trait (0) parameters were unknown and estimated from datasets of 100 and 500 examinees. rlype I error rate was unaffected by estimating item parameters from the data. Power was slightly lower in the 100-examinee datasets, but was almost identical in the 500-examinee datasets.
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