Population stratification

Population stratification is a form of confounding in genetic association studies that occurs when disease and non-disease populations include a different racial or ethnic mix. In this example, 60 percent of the light-skinned people have the red variant (regardless of eye color), and 80 percent of the dark-skinned people have the red variant (regardless of eye color). Hence, there is an association between skin color and the red variant but not between eye color and the red variant. However, there appears to be an association between eye color and the red variant because people who are light-skinned are more likely to have blue eyes and people with dark skin are more likely to have black eyes. Correction for this type of confounding is done by adjusting or stratifying the analysis.