Hi all,
From time to time the question comes up regarding performing
second-level (group) analyses on contrast images vs. t statistics
obtained from first-level (single-subject) analyses. The conventional
wisdom is that performing second-level statistics on the con* images is
more appropriate. This makes sense to me, as the contrast images reflect
effect size, and thus are testing whether the effect differs from 0
across the group.
However, is it technically inappropriate to use t statistics (or their Z
equivalent) for second-level analysis? What is the rationale either way?
And in particular, if it's not inappropriate, what would the
interpretation be?
One challenge that comes to mind is the interpretation of a non-zero
effect. For example, a group of subjects may all have a t statistic of
0.1. This is consistently greater than 0 (which is what a second-level
one-sample t-test would show), but none of us would consider a t value
of 0.1 particularly meaningful. This is in contrast to a parameter
estimate differing from 0, which is easily interpreted as there being a
significant effect across subjects.
This is obviously not an issue restricted to neuroimaging, but thus far
I've not found a discussion of the topic in any context. Any opinions
would be most welcome (as would relevant references)!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Center for Cognitive Neuroscience and
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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