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The power of an fMRI study depends on two principle issues: (1) the size of the effect relative to the group variance; and (2) the accuracy of the estimates in each individual. For general power calculations (e.g. to estimate #1), I use G*Power ( http://www.gpower.hhu.de/en.html). There are two ways to increase power in an fMRI study: (1) collect more trials/events; or (2) collect more subjects. You could compute the cost associated with each option ( http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2423281/). The tool for doing this can be found here:http://fmripower.org/ At some point, collecting more data in each subject will not improve the power because the estimate does not change. It is an empirical question of how many trials in a paradigm are needed to reach the point where the estimates won't change/don't change enough to influence the results. I'd go to the literature and see what other groups have done using similar paradigms. In my own work, I try to have at least 30 trials of each trial type. This was based on a early fMRI paper: http://www.ncbi.nlm.nih.gov/pubmed/11496120 One last thing to keep in mind. Power calculations can not be computed post-hoc for the current study. Power calculations can only be done for future studies. 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 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email. On Tue, Jun 10, 2014 at 7:40 AM, Mark <[log in to unmask]> wrote: > Sorry but hope someone can help for these questions. > > Also, how can I estimate the power of my experiments? I mean, how can I > decide the number of trials I should collect in my experiment to reach > sufficient power during analysis? > > Thanks again. > > Mark >