In practice, the modal fMRI sample size of 15 – 20 subjects often provides little power to detect anything but very large effects (Yarkoni, 2009). For example, a one-sample t test performed on 20 subjects at a statistical threshold of p < .001 (the modal threshold in the fMRI literature) has only 40% power to detect even a canonically ‘large’ effect of d = 0.8. For a correlational analysis, the same sample size provides only 12% power to detect an extremely large correlation of r = 0.5.
How many LOW and UNDER-powered imaging genetic studies (where genetic variation is merely one of many variables correlated with individual differences in task or baseline BOLD responses) have I have covered in this blog? Eeeek! I’d better not go there … I’ll go to ENIGMA instead.