________________________ Reply Separator _________________________________
Subject: Re: (Changes in binding & mood score)
Author: andrew at Internet-SHAR/dd.RFC-822=andrew\@fil\.ion\.ucl\.ac\.uk
Date: 9/25/98 02:21 PM
Dear Andrew,
thank you for your very clear explanation of mean-centering the covariate.
and for also explaining the reason for doing so (something that has
mystified me for some time).
However, might there be occasions where mean-centering wiould not be
appropriate?
I have made some (unsuccessful) attempts at doing covariate analyses using
measured blood concentrations of a drug X. (This happens to be a sedative
drug which we believe decreases global CBF. It produces multiple, very
large regions of significant change in blood flow.)
At pre-drug baseline, the measured concentration is of course zero.
After the drug, it might vary, say from 50 to 150 ng/ml.
In my attempted covariate analysis, I chose not to mean-center the
covariate values because I felt that a value of 0 was meaningful and should
not be changed to some other arbitrary value. Mean centering would lead
to spurious values of say -50 and +50 instead of true values of 0 and 100,
for a particular pair of scans.
Doing this covariate analysis my way led to very strange results, such that
the +1 covariate contrast identified regions of rCBF decrease, and the -1
contrast identified regions of rCBF increase. (See my prior message "When
is a positive correlation a negative correlation?" dated April 9, 1998)
It was later suggested by Dr. Paul Grasby that the inclusion of the
baseline scans with covariate = 0 had biased the output of the analysis.
He recommended that I redo the analysis using only post-drug scans.
However wouldn't this violate the general advice that all available scans
should be included in any analysis so that the estimate of error variance
will be correct?
Can this explain why, when I redo the covariate analysis omitting pre-drug
baseline scans, I get a complete absence of any significant voxels at a
corrected p of 0.05. (SPM95 analysis)
Can you advise how to proceed in this special situation?
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Ruth Reinsel Ph.D. e-mail: [log in to unmask]
Dept. of Anesthesiology
Memorial Sloan-Kettering Cancer Center
1275 York Avenue Tel: (212) 639-8111
New York, N.Y. 10021 Fax: (212) 772-8646
U.S.A.
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