This article develops a new cumulative sum (CUSUM) statistic to detect aberrant item response behavior. Shifts in behavior are modeled with quadratic functions and a series of likelihood ratio tests are used to detect aberrancy. The new CUSUM statistic is compared against another CUSUM approach as well as traditional person-fit statistics. A simulation study demonstrates the advantage of the proposed method. Also, the person-fit methods are applied to real response data from the administration of a high-stakes exam. The use of CUSUM charts to help visually identify types of aberrant behavior is demonstrated. (Contains 10 figures and 2 tables.)