I'm reading a paper that used whole exome sequencing on an African American and European populations to discover novel low frequency and rare variants associated with lipid levels & the risk of coronary heart disease.
They used the Illumina Human exome genotyping array that was designed based on coding variants discovered from sequencing the exomes of 12,000 individuals. Therefore, they selected their participants from a study who were not among the 12,000 individuals utilized to design the array .
My question is: Why would they select individuals who were not among the 12,000 used to design the array? (I'm thinking because maybe it could produce false positives associations? bias?)