Given the extent of DIF tests conducted by the number of patient characteristics and substances, relatively few instances of DIF were detected. However, where DIF occurred, it affected most or all of the criteria for that substance, rather than just a few criteria. DIF by race/ethnicity may occur in the cocaine criteria if cocaine and crack use are differentially associated with race/ethnicity, information unfortunately unavailable in our sample. DIF in the cannabis criteria may occur because of increased potency in available cannabis over time. This could lead to differences in the interest in or perception of this substance between younger and older patients, whose drug preferences become set at a relatively early age but during a different chronological period. We know of no explanation of the DIF by mood disorders for heroin criteria, but all of these issues merit investigation in future studies.
