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st: Using -ci- with weights


From   "Peter Jepsen" <pjspam@nisjepsen.dk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Using -ci- with weights
Date   Mon, 2 Jan 2006 21:57:45 +0100

Dear Statalisters,

I am doing a research project on the incidence of appendicitis over the past
ten years. The project is based on data from administrative registries, and
I am validating the recorded diagnoses by comparing with information in the
medical records. Appendicitis diagnoses originate from a number of
hospitals, and I validate 10 diagnoses per hospital. 
I calculate the positive predictive value for each hospital, with 95% CI,
using -ci- with the 'binomial' option, but I have problems calculating the
95% CI for all hospitals combined. The thing is, I would like to weigh each
hospital's positive predictive value with the hospital's contribution to the
total number of appendicitis cases. In other words, the validity of
diagnoses given in large-volume hospitals should matter more than the
validity of diagnoses given in hospitals with only few cases. 
Unfortunately, I am not sure how to do this. Using -ci- with the number of
cases per hospital as 'aweight' gives results that look reasonable to me
(although the upper limit exceeds 100% on a few occasions), but it seems
that I am in effect weighing with each hospital's variance, not the number
of cases. I am not really comfortable doing this (perhaps I would feel
better if I were a statistician), and I hope that you will give me some
ideas to a better approach, if there is one.

Thank you in advance for any assistance,
Peter.

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