The purpose of this study is to evaluate the extent to which item response theory (IRT) proficiency estimation methods are robust to the presence of aberrant responses under the "GRE"® General Test multistage adaptive testing (MST) design. To that end, a wide range of atypical response behaviors affecting as much as 10% of the test items were simulated using a generic GRE 2-stage MST and the 2-parameter logistic (2PL) IRT model. As expected, some differences were found among the 5 estimators in terms of the recovery of the true theta ability; for example, Bayesian estimators had lower error variance and their estimates were regressed to the mean.Once the IRT theta estimates were scaled onto a comparable reporting score scale, however, it was found that all the estimation methods investigated, including the one currently used to score GRE MSTs, were equally robust under the simulated conditions.