Abstract: Methods are provided which are useful in assessing drug response data, or response data to other types of clinical treatment, in a group of test subjects, and in predicting responses in other subjects, by defining a population genetic structure comprising one or more genetic clusters. The population genetic structure is obtained by analysis of the genotypes of genetic loci present in nucleic acid samples obtained from members of the group of test subjects. The population genetic structure provides an improved tool for assessing the results of a study, such as a clinical trial, into the effectiveness of a drug or other clinical treatment in different sub-populations as it is based directly on genetic criteria for assigning subjects to sub-populations.