A second important challenge to overcome in studies of epigenetic modification in the etiology of psychopathology is limitation by characteristics of the sample. Samples that are not adequately sized or that are not representative pose particular challenges to study validity. First, samples must be adequately sized to afford the study sufficient power. Studies with small samples, and therefore low power, increase the chances of Type II error, or false negative findings. More insidiously, however, small samples can also increase the chances of Type I error, increasing the likelihood of false positive findings.[38–40] The false discovery rate (FDR), the proportion of significant findings in a field that are actually the product of Type I error, given by equation 1, is a function of the specified Type I error rate (α), the prior probability that a hypothesis is correct, and the study power.
