This article presents the Variable Match Index (VM-Index), a new statistic for detecting answer copying. The power of the VM-Index relies on two-dimensional conditioning as well as the structure of the test. The asymptotic distribution of the VM-Index is analyzed by reduction to Poisson trials. A computational study comparing the VM-Index with the K-Index demonstrates that with the VM-Index, there is a large decrease in the Type II error rate and a smaller decrease in the Type I error rate.
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