Re: st: Same code, same machine, same data, different results

[Yuval Arbel <yuval.arbel@gmail.com>]

Mattia, your question reminds me an interesting anecdote I heard from
scholars who had the privilege to use University computers back then
in the 60's: each time they ran the program, they got different
outcomes.

Back then, the reason was an overflow of memory - under such
circumstances, the computer did not stuck or gave error message, but
simply gave wrong answers!!!

On Thu, Sep 6, 2012 at 1:29 PM, Knee, Alexander
<Alexander.Knee@baystatehealth.org> wrote:
> I've encountered this several times before.  In my case, I had a repeated measures dataset in long form sorted by time.  As it turned out, time was repeated within the same subject therefore every time I ran my code, the data would sort differently and give different results.  I discovered this by running my code until I got two different results and saving the data files.  I then used -cf- to compare the files.  Theoretically (in my mind) my data files should have been exactly the same, but -cf- pointed exactly to where the differences existed and hence my coding error.  This might be a good place to start.
>
> In another case I had a small dataset that I was trying to do too much with.  Again, it came down to how the data was sorted as to whether the model converged or not.
>
>
> Hope this helps!
> Alex
>
>
> ----
> Alex Knee
> Research Assistant Professor, Tufts University School of Medicine
> Biostatistician, Baystate Medical Center
> 280 Chestnut St., 3rd Floor
> Springfield, MA
> Alexander.Knee@baystatehealth.org
> Phone: 413-794-1086
> Fax: 413-794-0300
>
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of David Radwin
> Sent: Wednesday, September 05, 2012 5:50 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: RE: st: Same code, same machine, same data, different results
>
> Mattia,
>
> Is the exact same set of variables being dropped every time?
>
> David
> --
> David Radwin
> Senior Research Associate
> MPR Associates, Inc.
> 2150 Shattuck Ave., Suite 800
> Berkeley, CA 94704
> Phone: 510-849-4942
> Fax: 510-849-0794
>
> www.mprinc.com
>
>
>> -----Original Message-----
>> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
>> statalist@hsphsun2.harvard.edu] On Behalf Of Joerg Luedicke
>> Sent: Wednesday, September 05, 2012 2:24 PM
>> To: statalist@hsphsun2.harvard.edu
>> Subject: Re: st: Same code, same machine, same data, different results
>>
>> Think of it this way: you have some input (the csv file) which does
>> not change, then you execute something (your do-file), and finally use
>> -regress- to fit a model. Given that the input is fix, and -regress-
>> certainly produces the same results every time it is applied to the
>> same data, the problem _must_ lie in your do-file.
>>
>> J.
>>
>>
>> On Wed, Sep 5, 2012 at 5:10 PM, Mattia Landoni
> <mattia.landoni@gmail.com>
>> wrote:
>> > Dear statalisters,
>> >
>> > a friend of mine has a bizarre problem. She is running a regression as
>> follows:
>> >
>> > xi: regress a b c i.d i.e
>> >
>> > and her output is different every time. Has anyone ever seen a
>> > behavior like this? Below are some details.
>> >
>> > Environment:
>> > - Stata 11
>> > - Windows 32-bit
>> >
>> > Precise description:
>> > The do-file imports several files from .csv, then merges them, then
>> > runs the regression. If I run the do-file, I get certain results. If I
>> > issue the same regression command again, I get again the same results,
>> > as it should be. However, if I re-run the do-file from the beginning,
>> > I get slightly different results and the regression even reports a
>> > slightly different number of observations. (Say, 2663 vs. 2666). Every
>> > time all the data are taken afresh from the same static .csv sources.
>> > There is nothing random about the do-file, that I know. The xi:
>> > command generates about 200 i-variables and a few, maybe 10, are
>> > dropped because of collinearity. There are more than 2500
>> > observations.
>> >
>> > I could post the do-file here, but it's big and messy. If anyone has
>> > any insight after reading the above description, I'd be very glad to
>> > hear it.
>> >
>> > Thanks,
>> >
>> > Mattia
>> >
>> > --
>> > Mattia Landoni
>> > +1 217 4-A-QUANT (from US)
>> > +39 02 3206 21676 (from Italy)
>> > GMT -05:00 (US East)
>
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-- 
Dr. Yuval Arbel
School of Business
Carmel Academic Center
4 Shaar Palmer Street,
Haifa 33031, Israel
e-mail1: yuval.arbel@carmel.ac.il
e-mail2: yuval.arbel@gmail.com

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