In a few residential-psychotherapy units a joint outcome study is being conducted. One of its aims is the comparison of treatment outcomes. For this purpose it must be worked out whether the units treat the same types of patient. This was done by presenting 215 patients of 7 units a screening instrument for personality disorders (Questionnaire on Personality Traits, vkp). It was found that the populations of the units differed in composition. The differences match the expectations formulated earlier. The conclusion is that the vkp is a useful tool in this kind of comparative research.