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
>
>
>
>
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