THE EFFECTS OF SCORE GROUP WIDTH ON THE MANTEL-HAENSZEL PROCEDURE

Previous research examining the effects of reducing the number of score groups used in the matching criterion of the Mantel-Haenszel procedure, when screening for DIF, has produced ambiguous results. The goal of this study was to resolve the ambiguity by examining the problem with a simulated data set. The main results from this study call into question the preliminary recommendations of several other researchers that four or more score groups are sufficient and produce stable results. Although considerable stability and very little Type I error was noted with equal ability distribution comparisons, with unequal ability distributions, the Type I error rate was substantially inflated. These results argue against the appropriateness of implementing the procedure by collapsing score groups. The current data suggest that more than modest reductions in the number of score groups cannot be recommended when the ability distributions of the reference and focal groups differ.