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st: Bootstrapping: Comparing the difference between means


From   "Fleming, Fiona M" <f.fleming@imperial.ac.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Bootstrapping: Comparing the difference between means
Date   Wed, 3 Apr 2013 16:22:56 +0000

Hello,

I am hoping someone can help! I have been trying to work out, all day, how I can perform a bootstrap on the mean difference of two samples. I have scoured the Statalist, FAQ etc to get ideas and have recently been  trying to replicate the programming and commands illustrated in Example 2 on http://statistics.ats.ucla.edu/stat/stata/faq/ownboot.htm  with my own data set i.e.:

// step 1
ttest dcp  if (del==1) + (del==2), by(del)
return list
matrix mu = (r(mu_1), r(mu_2))
matrix list mu
                
// step 2
capture program drop mydiffa
program define mydiffa, rclass
preserve
bsample
ttest dcp if (del==1) + (del==2), by(del)
estat mu
return scalar diffa_1 = r(mu_1)
return scalar diffa_2 = r(mu_2)
restore
end

// step 3
simulate diffa_1=r(mu_1) diffa_2 = (r(mu_2), reps(100) seed(12345): mydiffa
bstat, stat (mu) n(200)
estat bootstrap, all

after running this 'simulate' I get the following error and cannot proceed- 
'' found where ':' expected
r(198);

                
Please could you offer any advice as to what I am doing wrong? 

In addition, what I have written above will only give me the bootstrap SE and CI for each of the means and not the difference of the means, if you have any further advice on how I might run that I would be extremely grateful.

Many thanks

Fiona Fleming
E-mail: f.fleming@imperial.ac.uk   





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