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Re: st: set seed and gsample


From   "Ben Jann" <ben.jann@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: set seed and gsample
Date   Sun, 11 Mar 2007 21:30:14 +0100

Okay, Nick. Of course your statement is the correct answer to the
quoted text. I already had some background information when I wrote my
"No". What Shige meant was to get the same sample each time the
do-file used to draw the sample is run. It is a multistage sample and
the do-file contains several -gsample- commands. Shige's colleague
misunderstood your advice and started to insert -set seed- commands
before each call to -gsample- although the seed should only be set
once at the beginning of the do file in this situation.
ben

On 3/11/07, Nick Cox <n.j.cox@durham.ac.uk> wrote:
I say No to your No.

Shige's question was

"I want to make sure I get the same sample each time
I invoke the gsample command".

Consider this:

. clear

. set obs 100
obs was 0, now 100

. gen i = _n

. set seed 280352

. gsample 20 , generate(sample1)

. gsample 20 , generate(sample2)

. assert sample1 == sample2
31 contradictions in 100 observations
assertion is false
r(9);

. set seed 280352

. gsample 20 , generate(sample3)

. assert sample1 == sample3

<NB: no response>

Perhaps Shige did not mean what he said, or
I am quoting out of context, but
but that quoted text was I what responding to.

Nick
n.j.cox@durham.ac.uk

Ben Jann

> Some days ago, the following issue concerning -gsample- was posted  by
> Shige on behalf of a  colleague:
>
> > I am trying to draw a PPS sample using the "gsample"
> command.  I want
> > to make sure I get the same sample each time I invoke the gsample
> > command by using the "set seed" command. However, even
> after I set the
> > random seed using "set seed" command, I still get different sample
> > each time. Has anybody encountered this problem?
>
> Inspection of Shige's colleague's do-file revealed that some -sort-
> and -bysort- commands were causing the trouble. It had nothing to do
> with -gsample-. -sort- has its own random number generator to break
> ties that does not depend on -set seed-. To make -sort- stable either
> specify the -stable- option or, better, add a -set sortseed- command
> at the beginning of the script (see -help sortseed-).
>
> Nick wrote:
> > You must -set seed- immediately before calling -gsample-.
>
> No. The seed should be set somewhere in the beginning of the do-file,
> before any command that might possibly depend on it (and it should
> only be set once - do not set the seed repeatedly in one script).

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