will occur when a low error rate is set, or when there are few true associations, or when the power is low. In genome-wide association scans, the number of true associations is expected to be small by comparison with the number of tests, so that the false discovery variance is relatively high in relation to the target rate, and the FDR approach may not be reliable for controlling the error rate within studies. In gene expression experiments, however, the number of true associations is somewhat higher and FDR methods are more appropriate for those studies.
