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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