Population stratification and ancestry variation can lead to spurious allelic associations in genetic studies. The common approach to control for population stratification in analysis is to include a series of principal components created from genotype data as covariates. Recent studies (Haworth et al, 2018) have highlighted that this approach may not be suitable as a method of control, particularly for regional studies such as ALSPAC.

Haworth, S. et al. Common genetic variants and health outcomes appear geographically structured in the UK Biobank sample: Old concerns returning and their implications. bioRxiv (2018).