Figure 1. Comparison of the frequency of the risk allele for four single nucleotide polymorphisms (SNPs) known to contribute to age-related macular degeneration (AMD) in the age-related eye disease study (AREDS) “control” and “questionable control” subjects. Each of these SNPs has been consistently associated with AMD in multiple studies. The figure shows that the questionable control category was similar to the control category in terms of the frequency of the risk allele. Questionable control subjects can be used as controls in analyses using the AREDS data set. Abbreviations: RCA is the regulation of complement activation locus containing the gene encoding CFH; please see introduction for other gene and locus names.