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st: repeat: predict after -streg-


From   Jeph Herrin <junk@spandrel.net>
To   statalist@hsphsun2.harvard.edu
Subject   st: repeat: predict after -streg-
Date   Thu, 12 Jul 2007 09:10:10 -0400

I posted this a few days ago, but got no replies;
thought I'd try again, shorter


My data are time to readmission (failure) after
hospitalization, with censoring on death:

-> stset ftime, failure(radm)

     failure event:  radm != 0 & radm < .
obs. time interval:  (0, ftime]
 exit on or before:  failure

------------------------------------------------------------------------------
   424787  total obs.
        0  exclusions
------------------------------------------------------------------------------
   424787  obs. remaining, representing
    95529  failures in single record/single failure data
 1.07e+07  total analysis time at risk, at risk from t =         0
                             earliest observed entry t =         0
                                  last observed exit t =        30

My model is

. streg `mycovars' , d(lnormal)

And my problem is:

 . predict median, median time
 . count if median<=30
    30

So, though the data have 95,000+ failures in less than 30 days, my
model predicts only 30 such failures.

I get the same results with other parameterizations, though lognormal
seems the best fit. Either I don't understand what the model is doing,
or I'm not calculating the correct predicted value. Should I expect
this?

Thanks in advance for any insights.


Jeph


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