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.

Best Regards, Donald McLaren ================= D.G. McLaren, Ph.D. Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School Postdoctoral Research Fellow, GRECC, Bedford VA Website: http://www.martinos.org/~mclaren Office: (773) 406-2464 ===================== This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (773) 406-2464 or email.

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

1. 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.

...

2. Is there a better way of comparing my subgroups to controls ?

Not that I can think of

...

Thanks.

Mahinda

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