Detecting answer copying when the regular response process follows a known response model

A statistical test for detecting answer copying on multiple-choice items is presented. The test is based on the exact null distribution of the number of random matches between two test takers under the assumption that the response process follows a known response model. The null distribution can easily be generalized to the family of distributions of the number of random matches under the alternative hypothesis of answer copying. It is shown how this information can be used to calculate such features as the maximum, minimum, and expected values of the power function of the test. For the case of the nominal response model, the test is an alternative to the one based on statistic ω. The differences between the two tests are discussed and illustrated using empirical results