Doug and Mahinda,
Since the assumption is that the slope is the same across all subjects, I would demean once. I would use one model with 1 control group and N patient group. The model will then test whether or not the covariate-adjusted group means are the same or different. By doing it in a single test, you will have more power since you will be using the pooled variance across all groups. You will also have the same slope estimate for all comparisons.
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On Mon, Nov 19, 2012 at 9:46 AM, Douglas N Greve greve@nmr.mgh.harvard.edu wrote:
On 11/19/2012 03:04 AM, Mahinda Yogarajah wrote:
Dear Experts,
I have a two groups - a control group and patient group. The patient group can also be divided into 4 subgroups (but not control gp). Both groups are evenly split between male and female. So far I have compared the patient and control groups as a whole (age and icv as continuous/nuisance variables) but now i want to compare each of the subgroups to controls. I have tried doing this in qdec very crudely by running 4 analyses in qdec where I exclude all the subgroup cases execpt for those belonging to the subgroup I am looking at. My questions are
- Is this valid (I would correct for the multiple tests) ? How does qdec cope with exclusions given that variables (age) have been demeaned across a whole group.
whether it is valid or not depends on what you are trying to conclude. If you want to compare across the tests, I would do the demeaning once across all subjects and not individually for each test.
- Is there a better way of comparing my subgroups to controls ?
Not that I can think of
Thanks.
Mahinda
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