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Re: st: applying sampsi: sub-groups

From   Ronan Conroy <>
Subject   Re: st: applying sampsi: sub-groups
Date   Thu, 7 Feb 2008 15:36:07 +0000

On 6 Feb 2008, at 22:08, Michael McCulloch wrote:

In my study design, subjects are randomized to one of two groups, treatment and control. Treatment group receives the new intervention, and control groups receives standard care.

My question is the following: after random allocation, within the treatment group subjects will be assigned to one of three treatment doses of the same treatment, depending on a differential diagnosis. In practice, in estimating sample size requirements, would one consider the three sub-groups as distinct groups, or aggregate them all into the treatment group?
This depends on the hypothesis of the study, not on any statistical consideration.

If the purpose of the study is to look at the relationship between dose and response, then the underlying model is a regression model of some sort with doses of 0 (control), 1, 2 and 3. (Substitute actual doses)

However, I have also encountered designs where the hypothesis was that the basic dose was sufficient, and that higher doses did not increase treatment effectiveness (in nutrition supplementation in pregnancy). In this case, your sample size for the first test (basic dose is sufficient) is all treated cases versus controls. A second dummy variable (whether the person received the higher dose) tests that second hypothesis, and the sample sizes here are for high-dose cases versus low-dose cases.

P Before printing, think about the environment
Ronan Conroy
Royal College of Surgeons in Ireland
Epidemiology Department,
120 St Stephen's Green, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95

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