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st: Bootstraped nadirs with 95% CI's for frac poly approximations


From   Jim Nicholson <nicholsj@mail.med.upenn.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: Bootstraped nadirs with 95% CI's for frac poly approximations
Date   Tue, 17 Aug 2004 12:30:41 -0400

New member with several questions.....

I am looking at data that relates continuous change in an outcome as a function of time (cesarean delivery is the outcome and the time is the day of gestational age in the term period of pregnancy when delivery occurs (37 weeks - 42 weeks gestation). Different numbers of patients have delivered on different days but the percentage of patients having a cesarean delivery per day seems to change continuously with increasing gestational age at delviery. I have been able to generate a weighted frac poly curve to approximate these relationships and I find that there is a low point in the curve - an optimal time of delivery!.

I have been able to capture this low point (nadir) within the frac poly curve by requesting the 1st percentile value. However, I would like to estimate the 95% CI around this point for both
the x and y coordinates (e.g. a 95% CI around both both gestational age and rate of outcome). I think this requires bootstrapping but I can't seem to make it work.

Any ideas?

Jim Nicholson,M.D.
Assistant Professor
Dept of Family Prac and Comm Medicine
University of Pennsylvania Health System

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