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st: RE: applying sampsi: sub-groups

From   "Lachenbruch, Peter" <>
To   <>
Subject   st: RE: applying sampsi: sub-groups
Date   Wed, 6 Feb 2008 15:38:31 -0800

In essence you have a 2x3 design.  You could only consider all subjects
on treatment as one group if there is no dose effect.  More is possible,
but you may need to specify what you want.  For example you could treat
doses as distinct effects and then you have six groups.  Or you could
consider a regression type model on your doses and have a regression
model.  Or you could think of a dose-response model with zero dose,
level 1, level 2 or level 3 doses.  You need to think of your analysis
now rather than after the data have been gathered.


Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001

-----Original Message-----
[] On Behalf Of Michael
Sent: Wednesday, February 06, 2008 2:09 PM
To: Statalist
Subject: st: applying sampsi: sub-groups

I am applying sampsi to a project design, and am puzzled by a 
question which perhaps one of the list members may have an opinion.

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?

Thank you.


Best wishes,
Michael McCulloch

Pine Street Foundation
124 Pine St., San Anselmo, CA 94960-2674
Tel:	(415) 407-1357
Fax:	(415) 485-1065
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