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RE: st: different result each time


From   "Rajesh Tharyan" <R.Tharyan@exeter.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: different result each time
Date   Thu, 25 Sep 2008 10:50:02 +0100

Hi Eva,

Thanks .. that was exactly the problem!  I was doing a merge way upstream
before the section where I run the t-tests, I was doing select all and
running the do file and that was causing the problem. I think with the
stable option that problem seems to have gone away.  

Thanks

Rajesh 
Tel: +44 (0)1392 262544
Fax: +44 (0)1392 262475
E-mail: r.tharyan@exeter.ac.uk

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Eva Poen
Sent: 25 September 2008 10:12
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: different result each time

Rajesh,

I find this puzzling. The usual reason for differing results is that
some of the code requires random numbers, and the seed had not been
set (e.g. -set seed 123-). However, there is nothing random in your
code.

The only thing I can think of is that there is a random element
happening before the loop that you don't show us. Do you -sort- your
data, and then use -in-? E.g.

sort thisvar thatvar
keep in 1/200
foreach i in 1 2 3 4 7 0 {
  ...
}

If your -sort- statement does not uniquely identify observations, you
will get a different set of 200 observations each time you run the
loop. This example shows it for the auto data:

***
forvalues x = 1/5 {
 sysuse auto, clear
 gen id = _n
 sort mpg
 list id in 6/10
}
***

Hope this helps,
Eva


2008/9/25 Rajesh Tharyan <R.Tharyan@exeter.ac.uk>:
> Hi all,
>
> I am using the following code to do a bunch of t-tests and output the
> results to excel
>
> foreach i in 1 2 3 5 7 9 {
> ttest winar`i' if type=="C", by(class)
> scalar m1`i'=`r(mu_1)'
> scalar m2`i'=`r(mu_2)'
> scalar mdiff`i'=(`r(mu_1)'-`r(mu_2)')
> scalar t`i'=r(t)
> scalar p`i'=r(p)
> }
> matrix C =
>
(m11,m12,m13,m15,m17,m19\m21,m22,m23,m25,m27,m29\mdiff1,mdiff2,mdiff3,mdiff5
> ,mdiff7,mdiff9\t1,t2,t3,t5,t7,t9\p1,p2,p3,p5,p7,p9)
> matrix rownames C= PB PS PB-PS t-stats p-val
> matrix colnames C= (per1) (per2) (per3) (per4) (per5) (per6)
> mat li C
> xml_tab C,append
> clear all
>
> Each time I run this I get different results. See below ( only the first
row
> of matrix is shown)
>
> Run1
> PB      0.234   0.109   0.027      -0.020     0.006     0.186
>
> Run2.
> PB   .22601096   .10431468   .02967896  -.00025647   .02184916   .21296798
>
> And so on. Why? Any thoughts
>
> Regards
> rajesh
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