Cases were all diagnosed by a rheumatologist as having psoriatic arthritis; most were evaluated using CASPAR criteria. Controls were at least 18 years of age and unaffected with either psoriasis or psoriatic arthritis. After genotyping on the Illumina array, we applied the usual quality controls measures to the samples. Samples were excluded if they represented a population outlier with substantial non-European admixture (determined with PCA), were duplicates or first or second degree relatives of other samples, had a genotype call rate < 98%, had outlier values for inbreeding coefficient ≥ 1.5 times the interquartile range beyond the lower or upper quartiles (F < -0.0225 or F > 0.0285), or were part of unresolvable sample swaps determined by X chromosome genotypes.