The Mantel-Haenszel (MH) procedure is commonly used to detect items that function differentially for groups of examinees from various demographic and linguistic backgrounds--for example, in international assessments. As in some other DIF methods, the total score is used to match examinees on ability. In thin matching, each of the total score points is used as its own matching category, whereas in thick matching the total score is discretized into several score ranges. Evidence regarding how matching type affects the accuracy of MH procedure is inconclusive. The current study investigated the effects of thin and thick matching in conjunction with sample size, purification, symmetric and asymmetric groups sample sizes, test length, and differences in the ability distributions. Results suggest that whenever feasible, purification should be used in conjunction with thin matching.