The MGMD takes the GDM and integrates it into the framework of discrete mixture distribution models for item response data (see von Davier & Rost, 2006). This increases the utility of the GDM by allowing the estimation and testing of models for multiple populations. The MGMD allows for complex scale linkages that make assessments comparable across populations and makes it possible to test whether items function the same in different subpopulations. This can be done with known subpopulations (defined by grade levels, cohorts, etc.), as well as with unknown subpopulations that need to be identified by the model.