R help - Feb 2013 - Why R simulation gives same random results?

Hi, list
I am doing 100,000 iterations of Bayesian simulations.

What I did is I split it into 4 different R sessions, each one runs 25,000
iteration.  But two of the sessions gave the simulation result.

I did not use any set.seed().  What is going on here?

Thanks,
Mike

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> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of C W
> Sent: Tuesday, February 19, 2013 5:32 PM
> To: r-help
> Subject: [R] Why R simulation gives same random results?
> 
> Hi, list
> I am doing 100,000 iterations of Bayesian simulations.
> 
> What I did is I split it into 4 different R sessions, each one runs
> 25,000
> iteration.  But two of the sessions gave the simulation result.
> 
> I did not use any set.seed().  What is going on here?
> 
> Thanks,
> Mike
> 
I may have missed it, but I haven't seen a response to this question yet. 
If no one has responded, it is probably because you haven't provided the
code you used to run the simulation, or a reproducible example which
demonstrates the problem.  You say you didn't use set.seed(), but you
didn't show what you did do, so there is insufficient information for anyone
to be able to comment what went wrong.

Dan 

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

To know for sure we need to know how you are running these different R
sessions, but here are some possibilities:

The help page for "set.seed" says that if no seed exists then the seed
is
set based on the current time (and since 2.14.0 the process ID).  So one
possibility is that 2 of the sessions are started close enough together
that they get the same seed.  Or the difference in time and process ID
cancel each other out.

Another possibility (also mentioned in the help page) is that if the seed
was saved in a previous session then it will be restored in the new
session, if all the sessions are reading in the same stored session (or
just the 2 that are the same) then they would start from the same seed.


On Tue, Feb 19, 2013 at 6:31 PM, C W <tmrsg11@gmail.com> wrote:
> Hi, list
> I am doing 100,000 iterations of Bayesian simulations.
>
> What I did is I split it into 4 different R sessions, each one runs 25,000
> iteration.  But two of the sessions gave the simulation result.
>
> I did not use any set.seed().  What is going on here?
>
> Thanks,
> Mike
>
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>
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>


-- 
Gregory (Greg) L. Snow Ph.D.
538280@gmail.com

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