Thank you, Dr. Peele. That's quite helpful.
Clifford Workman
Senior Research Program Coordinator
Division of Geriatric Psychiatry and Neuropsychiatry
Johns Hopkins University School of Medicine
Johns Hopkins Bayview Medical Center
5300 Alpha Commons Drive, Fourth Floor
Baltimore, MD 21224
(410) 550-9052
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-----Original Message-----
From: Jonathan Peelle [mailto:[log in to unmask]]
Sent: Thursday, September 22, 2011 11:30 AM
To: Clifford Workman
Cc: [log in to unmask]
Subject: Re: [SPM] Question about subtraction images
Dear Cliff,
> I've noticed that, when looking at longitudinal PET scans, some opt to
> use subtraction images and one-sample t-tests to look at change across
> two time points, in lieu of paired-t tests. I am wary of this, and was
> wondering if anyone could comment on the use of subtraction images. Thanks so much.
A one sample t-test on difference scores (aka subtraction images) is the same as a paired t-test across those images. To convince yourself of this you can generate some example data in Matlab and try some
t-tests:
% generate some data
time1 = 5 + 2*randn(1,15); % mean = 5, SD = 2
time2 = 8 + 2*randn(1,15); % mean = 8, SD = 2
% calculate a difference score
diff = time2 - time1;
% t-tests
[h,p,ci,stats] = ttest(time1, time2);
[h1,p1,ci1,stats1] = ttest(diff);
% results
fprintf('\n');
fprintf('Paired t-test: t(%i) = %.3f, p = %.3f\n', stats.df, stats.tstat, p); fprintf('One sample t-test: t(%i) = %.3f, p = %.3f\n', stats1.df, stats1.tstat, p1);
Hope this helps!
Jonathan
--
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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