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st: sw command within bstrap


From   pra06 <e.thomas@cphc.keele.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   st: sw command within bstrap
Date   Fri, 4 Feb 2005 11:45:29 +0000

Hi all

I am embarking on a project to derive a clinical prediction rule (using 
logistic regression) and it has been suggested that I should use a 
bootstrapping technique to study the internal validity of the final prediction 
model in order to derive bias-corrected coefficients/odds ratios.

Perusing the manuals I came across the bstrap command which seems like a good 
place to start. I have been able to use a simple logistic regression in the 
bstrap command to calculate bias-corrected coefficients/odds ratios. However, 
I have seen articles that have suggested that they have used a stepwise 
command within the bootstrapping routine, ie replicating the exact same 
procedure that I will used to arrive at the final prediction model in each of 
the 1000 samples. I am unable to get bstrap to implement the sw command.

I know that I can simply derive 1000 samples and apply the sw procedure to 
each sample. However, I am unsure as to how to assimilate the data from these 
1000 samples as not only will the magnitude of the coefficients vary in each 
sample (simple part!) but the actual variables chosen by each sw procedure may 
be different in each sample.

Any help with  programming in Stata or detailed examples in the literature 
(many just glibly state that this process was carried out  very helpful!) 
would be appreciated

Best wishes
Elaine Thomas


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