European Americans are …a structured population due to historical immigration of diverse source populations. Discerning the ancestry of European Americans genotyped in association studies is important in order to … identify genetic variants whose contribution to disease risk differs across European ancestries.

…analyzing 4,198 samples from four genome-wide association studies ..shows components roughly corresponding to Northwest European, Southeast European, and Ashkenazi European ancestry are the main sources of European American population structure.

via PLOS Genetics: Discerning the Ancestry of European Americans in Genetic Association Studies.