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
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On Mon, May 5, 2014 at 10:57 AM, Eryilmaz, Huseyin Hamdi <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



-- 

Hamdi Eryilmaz, PhD

Massachusetts General Hospital
A.A. Martinos Center for Biomedical Imaging
Brain Genomics Laboratory
149 13th St, Charlestown, MA 02129

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