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st: Re: sampling question


From   "Michael Blasnik" <michael.blasnik@verizon.net>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: sampling question
Date   Wed, 10 Mar 2004 14:38:17 -0500

You will probably be best off using a loop and a postfile.  I've done this
kind of thing farily often, here's an ado that should work:

program define prob1
version 8.2
syntax using , saving(str) [reps(integer 100) samples(integer 30)
samplen(integer 100)]
tempname post
local tokeep=`samples'*`samplen'
postfile `post' b1 b2 using `saving'
set seed 1234567
forval rep=1(1)`reps' {
use `using', replace
gen rand=uniform()
sort rand
qui keep in 1/`tokeep'
gen int sample=int((_n-1)/`samplen')+1
collapse y x1 x2 , by(sample)
qui reg y x1 x2
local beta1=_b[x1]
local beta2=_b[x2]
post `post' (`beta1') (`beta2')
}
postclose `post'
use `saving', replace
end


This ado is hardcoded for y x1 and x2 as the variables, but allows you to
specify the number of overall replications, the subsample sizes and number
of subsamples for each run.  It also specifies a seed to provide
repeatability.  You could use it like this:

prob1 using mydata, saving(myresults)


Michael Blasnik
michael.blasnik@verizon.net

----- Original Message ----- 
From: "ACHINTYA RAY" <achintyar@hotmail.com>
To: <statalist@hsphsun2.harvard.edu>
Sent: Wednesday, March 10, 2004 1:58 PM
Subject: st: sampling question


> Sorry for posting a somewhat long query. I'll appreciate if you can help
me
> perform the following operation is Stata. Bootstrap commands in Stata
helps
> me perform some of the things. Specifically, I am looking for a solution
> where (sort of) bootsrap is allowed within a bootstrap setup.
>
> Suppose I have a sample of 20,000. I have 3 variables: y, x1 and x2.
> Step 1: Draw 30 samples of size 100 each.
> Step 2: Calculate mu(y), mu(x1), mu(x2) for each of the subsamples.
> Step 3: Create a matrix of mu(y), mu(x1), mu(x2). Clearly, it will be a
> matrix of dimension 30x3. Let's call it [Y X1 X2]
> Step 4: Run the model Y=a + b1*X1 + b2*X2 + e
> Step 5: Create file to save estimated b1 and b2.
> Step 6: Repeat Steps 1 through 5 several times (100 times) to obtain a
> matrix of estimated b1 and b2s (100x2 in dimension).
>
> While I can create a grand file of all the means, I am having difficulty
in
> creating the subroutine described in Steps 1 through 5.
>
> Thanks
>
> Achintya Ray
> Senior Research Associate
> Department of Medicine
> Vanderbilt University
> achintya.ray@vanderbilt.edu
>
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