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st: odd results after insample


From   "Carlo Lazzaro" <carlo.lazzaro@tin.it>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: odd results after insample
Date   Sat, 26 Sep 2009 14:59:58 +0200

Dear Statalisters,
as an alternative to - simulate - , I have written the following do file
(for Stata 9.2/SE) to draw 2000 random samples, 20 observations each, from a
normal distribution: 

drop _all
set more off
set obs 2000
obs was 0, now 2000
g double ln_g_20=.
g double ln_sd_g_20=.
set seed 999
qui gen A=5.37 + 1.19*invnorm(uniform()) in 1/972
qui forvalues i = 1(1)2000 {
qui gen ln_20`i'=A
qui generate random`i' = uniform() 
qui sort random`i'
qui generate insample`i' = _n <= 20
qui sum ln_20`i' if insample`i' == 1
replace ln_g_20=r(mean)  in `i'
replace ln_sd_g_20=r(sd) in `i'
drop ln_20`i'
drop random`i' 
drop insample`i'
}
drop A

However, as a result I have obtained 1721 observations instead of the
expected 2000. 

sum ln_g_20 ln_sd_g_20

Variable |       Obs        Mean    Std. Dev.       Min        Max
-------------+--------------------------------------------------------
     ln_g_20 |      1271    5.314033    .3800687    3.79247   6.587941
  ln_sd_g_20 |      1271    1.101084    .2835007   .0260279   2.161299


Besides, results are even more puzzling when I increase the number of
samples (again 20 observations each), in that I get a different number of
observation for ln_g and ln_sd_g.

Comments are gratefully acknowledged.

Thanks a lot for your kindness and for your time.

Kind Regards,
Carlo


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