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Re: st: applying sampsi: sub-groups
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
[email protected]
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
http://www.flickr.com/photos/ronanconroy/sets/72157601895416740/
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