Thanks Donald for the clarification. In our design, for the condition A (errors in a memory task), number of trials depends on subjects' performances and therefore we've got some between subject variability there. If the number of trials is not considered and as long as it is sufficient to produce a stable estimate for the mean response, I guess we don't need to worry about this issue. Nevertheless, we probably need to exclude subjects with very few errors from the group analysis...
Best, Hamdi
From: freesurfer-bounces@nmr.mgh.harvard.edu [freesurfer-bounces@nmr.mgh.harvard.edu] on behalf of MCLAREN, Donald [mclaren.donald@gmail.com] Sent: Monday, May 05, 2014 11:20 AM To: Freesurfer support list Subject: Re: [Freesurfer] comparing two conditions with disproportionate number of trials
For the contrast A>B, the number of trials is not considered. When you contrast A and B, you are assessing the mean response to trials of condition A to the mean response to trials of condition B. You wouldn't want to weight this by the number of trials as then the contrast would not make sense.
The number of trials can effect the accuracy of the estimate of the mean response to a condition. In this way, A could be stronger or weaker than it actually is in each subject. Because of the decreased accuracy, you could also have increased variability between subjects.
As a result of the increased variability, the mean response needed for A to be different from 0, will be greater than that in B. However, this not imply that A will be greater than B as the significance of A doesn't tell you anything about A>B. In fact, you could construct a case where B>A and where A>0 is significant, but B>0 is not significant. In A vs B, you are using the within-subject variance and fro A or B vs 0, you are using the variance between subjects.
Generally, you need 30-40 trials to get a stable estimate of the mean response of each condition.
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 Mon, May 5, 2014 at 10:57 AM, Eryilmaz, Huseyin Hamdi <HERYILMAZ@partners.orgmailto:HERYILMAZ@partners.org> wrote: Dear FS experts,
I have a question about the way contrasts work in an event-related design. I am wondering if freesurfer is doing some sort of weighing when it compares two different conditions with incomparable number of trials. For example condition A is represented by 10 trials in the paradigm file, whereas condition B consists of 80 trials. In that case, for the contrast A>B does freesurfer compensate for the small number of trials in condition A? If so, could it lead to inflated activations? We suspected this as in our results, the condition with fewer trials depicted strong activations.
Thanks very much for the tips!
Best, Hamdi
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Hamdi Eryilmaz, PhD
Massachusetts General Hospital A.A. Martinos Center for Biomedical Imaging Brain Genomics Laboratory 149 13th St, Charlestown, MA 02129 Phone: +1 617 643 7462tel:%2B1%20617%20643%207462
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