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st: strange confidence intervals in survival analyses


From   Paula Brentlinger <brentp2@u.washington.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: strange confidence intervals in survival analyses
Date   Mon, 16 Jun 2003 18:14:31 -0700 (PDT)

Dear all -

     I am trying to calculate mortality rates using Stata 7.0, but am having a problem with confidence intervals.   In my survey data set, I have a dichotomous outcome variable representing death, and a second variable representing person-time.  I have been using "stset" to convert the data into survival-time, and "stptime" to calculate mortality rates.  This all seems to work very well except when I add p-weights.  The confidence intervals I get for the weighted estimates seem, well, improbable:  e.g. an estimate of 8.2 is reported to have a 95% CI of 3.0, 3.0. This problem is especially common in analyses with low numbers of deaths. 
     Has anyone else encountered this problem?  Is the problem mine or Stata's?
     Many thanks.

                                   Paula Brentlinger 

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