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st: predict after -streg-


From   Jeph Herrin <junk@spandrel.net>
To   statalist@hsphsun2.harvard.edu
Subject   st: predict after -streg-
Date   Tue, 10 Jul 2007 13:07:56 -0400

This is either a Stata question or a stats question,
apologies if it turns out to be the latter.

I am using -streg- to estimate a parametric survival
model with a lognormal distribution. The data represent
readmissions to hospital after a discharge, with readmission
being considered a failure. All patients (n=424,787) are
followed for 30 days, with approximately 25% readmitted
within that time.

-> 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 has 30+ covariates representing demographic and
comorbid characteristics of the patients. I would like
to get a predicted time to readmission for each patient
in the cohort. I will need to specify shared frailty by
hospital but for now am working with a standard model.

What is bothering me is that while 95,000 patients are
acutally readmitted prior to 30 days, when I calculate
the predicted time to failure:

 . predict median, median time
 . gen prd30 = median<=30
 . count if prd30
    30

Why the huge difference? Diagnostics show the lognormal
model fitting quite well, with the residuals falling right
along where they should. 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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