The previous work by Davis and colleagues involved about 50% fewer OCD cases and the variance in any estimate is a direct function of sample size. It is also possible that the Davis study had a different distribution of distantly related individuals than our relatively homogeneous sample from Sweden. Accuracy of SNP-based heritability diminishes as the fraction of very distantly related pairs, relative to all relative pairs, increases. Consistent with estimates from both studies being noisy, when we combined the Davis et al. results to obtain new estimates of average heritability per allele bin and heritability per chromosome, the average fit expectation was better than in either study alone.
