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

Previous research examining the effects of reducing the number of scor e groups used in the matching criterion of the Mantel-Haenszel procedu re, when screening for DIF, has produced ambiguous results. The goal o f this study was to resolve the ambiguity by examining the problem wit h a simulated data set. The main results from this study call into que stion the preliminary recommendations of several other researchers tha t four or more score groups are sufficient and produce stable results. Although considerable stability and very little Type I error was note d with equal ability distribution comparisons, with unequal ability di stributions, the Type I error rate was substantially inflated. These r esults argue against the appropriateness of implementing the procedure by collapsing score groups. The current data suggest that more than m odest reductions in the number of score groups cannot be recommended w hen the ability distributions of the reference and focal groups differ .