Differential item functioning (hereinafter referred to as DIF) is understood to be present when something about the characteristics of a test taker interferes with the tight relationship between ability and item responses. If a test taker has a particular ability level, they should be just as likely as any other test taker of the same ability to respond to a particular item in a specified way.

If two test takers have the same ability level, then their chances of responding correctly to a particular test item should not be altered by membership in whatever group is considered.

The finding of DIF means that some group membership has interfered with the tight relationship needed between ability level and chances of success for the item.

The authors of this program are a general internal medicine fellow, a biostatistics graduate student, and a biostatistics professor, respectively. We hope that this program helps to advance the detection of item bias in both the medical and the educational fields. We plan to write a similar program in SPSS soon in order that these advances are more accessible to those who do not use STATA.