Many GWA genotyping platforms are available, meaning that publicly available samples may not always have been genotyped for the same set of SNPs. However, imputation techniques [Marchini et al., 2007] allow us to combine samples, irrespective of genotyping platform, by making use of reference samples of haplotypes, such as those in the International HapMap project [The International HapMap Consortium, 2005], and simple models of population genetics to approximate the distribution of genotype cells carried by individuals not directly genotyped on the GWA array. In this way, samples that are directly genotyped for one SNP can be combined with samples for which the same SNP has been imputed, again leading to an increase in power.
