It entirely depends on the effect size. The smaller the effect size
you are looking to detect, the more subjects you will need. The larger
the effect size, the fewer subjects you will need. One can't say
universally that the effect sizes will be larger for one modality or
the other.
Most groups estimate the effect size, compute the number of subjects
required to get 50-80% power, and then conduct the study with that
number of subjects.
You can find more about power from: http://mumford.fmripower.org/
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Mon, Dec 10, 2012 at 1:29 AM, Jon Brock <[log in to unmask]> wrote:
> Hi,
>
> My impression from the literature is that (rightly or wrongly) 20-30 per group is an acceptable sample size for FMRI studies but that VBM studies require considerably larger numbers to have similar face validity. Could anyone shed some light on this for me? Is this a real phenomena (FMRI is more sensitive compared to VBM) or are other processes at work here?
>
> Cheers
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