part be due to the difference in geographic locations of the source populations which were sampled, as people from distinct regions tend to have different genetic background. For example, by using the Kruskal-Wallis test [29], which is the non-parametric version of the ANOVA test, we find that several major PCs in each study have significantly different distributions across different geographic locations (defined by either the recruitment center in the PLCO prostate cancer study, or the state of residence in the NHS breast cancer study).
