Thanks for your comments Michael.
Does non mean centered DODS extrapolate back to zero? I thought it appeared to
be extrapolating back to the beginning of the dataset (i.e. the smallest value
of the covariate of interest, in this case age).
Mean centering appears to reduce the significance map to a DODS analysis, I
think (although I find this all very confusing) because in DOSS one is just
looking at differences between groups at mean age? I am assuming that it is not
a wild coincidence.. and I understand that DOSS constrains groups to be of the
same slope.
Another observation is that the 'group differences' significance map of DODS, without mean centering the age, is highly similar to the 'thickness-age correlation difference' map of mean centered age DODS analysis. This also makes sense to me, however I
It is clear that mean-centering changes the meaning of the analysis. However I
have read some old mailing list threads in which it is not recommended.
However, if I just want to remove possible age interactions, I would have
thought that mean-centering is the best approach. Incidentally, both groups are
highly similar in mean, median range and s.d. of age. The age range being 63-23
is some of the reason for my cautious approach and given that foreach of my
groups n=15 I do not have much freedom in terms of looking at smaller age
groups.
I have a final question: the DOSS with age covaried (and DODS with mean-centred age) comparison between patients and controls is almost identical to a straight (no covariates considered) analysis between patients and controls. Can I necessarily conclude that age is not a significant factor in this analysis?
Any comments and corrections welcomed.
Many thanks,
Rysia
-----Original Message-----
From: Michael Harms [mailto:mha...@conte.wustl.edu]
Sent: 13 January 2009 15:01
To: Burmicz, Ryzarda
Cc: freesurfer@nmr.mgh.harvard.edu
Subject: Re: [Freesurfer] DODS/DOSS choice in patient/control study
Hello Rysia,
I'll add a comment by calling your attention to one statistical
"subtlety". When using a DODS ("separate slopes") model in the presence
of a continuous covariate, and then examining the significance of the
group effect, the interpretation (and results) depend critically on the
meaning of "zero" of the continuous covariate (e.g., whether the
covariate is centered or not). In your case, I see that you are using
age as the covariate (a very common covariate). If you do not center
the age variable, the resulting test of the group effect is in effect
whether the estimated intercepts, extrapolated back to age 0, differ
significantly. If you center the age variable (say around the mean
age), the test of the group effect is still whether the "intercept"
differs between groups, but now the intercept is centered at the mean
age along the x-axis.
If you experiment with different ages as the reference point (e.g.,
centered around 20, 40, 60 years), you will find that the DODS results
for the group effect can change dramatically.
Incidentally, when using a DOSS model, centering is irrelevant, because
the regression lines for the two groups have (by design) the same slope,
and thus a constant vertical difference, regardless of the location of
"zero" along the x-axis.
cheers,
Mike H.
On Tue, 2009-01-13 at 14:16 +0000, Burmicz, Ryzarda wrote:
Hello,
We are looking at patients versus control group analysis. We have done
a straight analysis and are now looking at covariates. DOSS produces
similar results to a straight patient vs control analysis, in that
some blobs are located in the same cortical regions. In the case of
DODS, there is no similarity whatsoever to the straight analysis
maps... which is why I am so stuck in deciding, because it really does
impact on the results quite drastically.
I understand the difference between DODS and DOSS – and I know the
following question is not an easy one to answer, however we have
sample sizes of 15 patient vs 15 control subjects and would like to
know if that is great enough such that we are better off going for
DODS.
There is no reason to say that the patient cortical thickness – age
relationship is any different in slope from that of normal ageing
generally; however in certain cortical regions patients exhibit a
thickening of the cortex, and thinning in other (albeit fewer)
regions.
Do you have any comments? I would have thought that our sample sizes
are rather small, but would appreciate your input.
I attach the comparison of patient vs control covaried for age under
both DODS and DOSS to illustrate the differences.
Thanks!
Rysia
Ryszarda Burmicz
Research Assistant
Centre for Neuroimaging Sciences
PO89
Institute of Psychiatry
_______________________________________________
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
_______________________________________________
Freesurfer mailing list
Freesurfer@nmr.mgh.harvard.edu
https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer
