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st: interim analysis in epidemiologic studies


From   Ricardo Ovaldia <ovaldia@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: interim analysis in epidemiologic studies
Date   Thu, 7 Aug 2003 07:02:27 -0700 (PDT)

Dear all;

I know that interim analysis are used in clinical
trials often with a "stopping rule" so that a trial
can be stopped if participants are being put at risk
unnecessarily. However, I am uncertain about the
effect of multiple looks of the data on the p-values
of the final analysis in epidemiologic studies such as
case-control studies. Two scenarios come to mind:

1. A case control study in which we are enrolling a
fix number of subjects (say 100 cases and 100
controls). If I compute odds ratio when I have
enrolled 25%, 50% and 75% of subjects does that affect
my p-value when I have 100% of the subjects? I do not
think it does because I set my final sample size to a
given value at the beginning.

2. Assume that now my sample size is not fixed but I
decide to enroll patients for a fixed amount of time,
say a year. How do the interim analysis affect the
final p-values?

Note that I am not using a stopping rule in either
case. Your thoughts will be much appreciated.

Thank you in advance,
Ricardo.


=====
Ricardo Ovaldia, MS
Statistician 
Oklahoma City, OK

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