The probability of test takers changing answers upon review of their initial choices is modeled. The primary purpose of the model is to check erasures on answer sheets recorded by an optical scanner for numbers and patterns that may be indicative of irregular behavior, such as teachers or school administrators changing answer sheets after their students have finished the test or test takers communicating with each other about their initial responses. A statistical test based on the number of erasures is derived from the model. Besides, it is shown how to analyze the residuals under the model to check for suspicious patterns of erasures. The use of the two procedures is illustrated for an empirical data set from a large-scale assessment. The robustness of the model with respect to less than optimal opportunities for regular test takers to review their responses is investigated. (Contains 5 figures.)