IRT modeling can also be used to assess differences in item functioning by sex and age. That is, do the items that are used to measure SUD mean the same thing for men and women and at during adolescence, young adulthood and late adulthood? An application of this type of modeling to nicotine dependence items showed that by allowing for limited measurement differences—due to differential sensitivity of items by sex—inferences about heterogeneity of heritability by sex at the latent factor become more meaningful. In this application, for example, individual items in the scale are somewhat more discriminating of the level of nicotine dependence in men than in women35. An item response framework is to be preferred over a sum score approach in which differential item functioning would be obscured and each item weighted equally regardless of its correlation with the latent construct to be measured. Finally, as was shown previously in using the CCC model to estimate the heritability of nicotine dependence, and repeated in the IRT analysis, it is important to include initiation to obtain unbiased estimates of the
