One of our major design considerations was sample size. We set out to include samples larger than those previously examined for genome-wide association, and our results suggest that such large sample sizes were necessary. Even with 2,000 cases and 3,000 controls, adequate power is restricted to common variants of relatively large effect (see Supplementary Table 2). We carried out an experiment to see which SNPs showing strong evidence of association in the full data (that is, signals outside MHC with trend test P<5×10-7), would have been detected at that same threshold in only a subset of our data (Fig. 6). Because it focuses on a particular but arbitrary P-value threshold, some care is needed in interpreting the figure. Nonetheless, for subsamples of 1,000 cases and 1,000 controls, of the 16 loci detected in the full study, we would have been certain of seeing only 2, with an expectation of about 6; for subsamples of 1,500 cases and 1,500 controls, we could expect to have seen about 9. These figures provide stark evidence that the larger the study sample, the more
