To follow up on this question, we were able to calculate effect sizes for every vertex on the surface using the gamma(var).mgh files, as well as an average effect size across each hemisphere surface using the equation below. In determining alpha, would the most appropriate method be to use the FDR-corrected alpha level that results from our pilot analysis? For example, for a two-group comparison we FDR at 0.05, which results in a corrected p (alpha) of 0.001, and we use that for the power analysis calculation (#3 below), thanks.

On Mar 22, 2011, at 4:55 PM, Douglas N Greve wrote:

For the power analysis, you need four things:
1. Effect Size
2. Number of Subjects
3. Target False Positive Rate (alpha)
4. Target False Negative Rate (beta)

Given any 3, you can compute the 4th. You can get the Effect Size from
the output of the QDEC analysis. Each analysis creates a GLM directory,
and there is a directory for each contrast in the GLM dir. In the
contrast dir, you will find several files, but the important ones for
this are the gamma.mgh and the gammavar.mgh. The gammavar is the square
of the error bar (ie, t=gamma/sqrt(gammavar)). The gamma will not change
as you add subjects (at least in expectation). The gammavar will drop
linearly with the number of subjects (again in expectation). The effect
size will then be gamma/sqrt(gammavar*Npilot), where Npilot is the
number subjects in the pilot study. In a new study with Nnew subjects,
the expected t will be tnew = gamma/sqrt(gammavar*Npilot/Nnew)

doug

Emily Rogalski wrote:
How would you complete a power analysis for a whole brain group
comparison qdec analysis?

Thanks,
Emily

--
Emily J Rogalski, PhD
Assistant Research Professor
Northwestern University
Cognitive Neurology and Alzheimer's Disease Center (CNADC)
320 E Superior Street
Searle Building 11th Floor
Chicago, IL 60611
312-503-1155 phone
312-908-8789 fax
erogalski@gmail.com <mailto:erogalski@gmail.com>
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Douglas N. Greve, Ph.D.
MGH-NMR Center
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