A question on exclusion of study participants for an Exome genotyping array
When you design the arrays, you need to have probes on the surface complementary to the sequence you want to detect. Depending on what you want to detect, you need to design these probes with known sequence on a known position.
If you want to detect single nucleotide polymorphisms (SNP), then you need a library of known SNPs on your ChIP, which are basically the position of the SNP and the surrounding sequence.

SNPs can be roughly grouped into two subgroups, common (I call them like this) and rare (frequency in the population lower than 0.1%). The problem with this ChIP based method is that it can only detect SNPs which are already known (or at least are located close to known SNPs with which they in linkage disequilibrium). So if you want to detect rare SNPs in your population sample, you need a big group of people from which you take the SNPs into you ChIP design. If you look at the numbers, one rare variant will only be present in about 12 of the 12.000 persons used in the design.

Your study group is then different from the group of people, which where used to make the ChIP design. Here you take people with a certain background (high lipid levels for example) and then compare, if people which had coronary heart problems have SNPs in common which might be connected to this disease. This can help to identify a mutation in a protein which is the risk factor.