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

[Jim Nicholson <nicholsj@mail.med.upenn.edu>]

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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