Jason, John, Thanks for the information. I am writing most of my syntax on PSPP on my Linux box but I will check it against a copy of SPSS I have access to periodically. Reproducability of the sample selection is important and I will see if I can get the same results from both SPSS and PSPP when I use the same seed across both systems.
I am also going to see if I can reproduce the same sample outcome using R. Should make for an interesting experiment. The only copy of SPSS that I have regular access to is Version 11, which may or may not make a difference. If I can get my hands on a newer copy of SPSS, I'll test it there too. Thanks --andy On Wednesday, September 21, 2011 06:29:01 PM John Darrington wrote: > In SPSS, you can choose between the MC rng and the Mersenne Twister > using the commands: > > SET RNG=MC. > > or > > SET RNG=MT. > > > See the SPSS documentation for details. > > > PSPP doesn't implement this. (Perhaps we should?) Like Jason says, we > always use the > Mersenne Twister. > > I also notice that the SPSS docs say that the default seed is 2000000 whereas > we set it > from the realtime clock. > > I'd be interested to see some of your experiements to see what is necessary > to make them > match (if it's at all possible). It would also be interesting to see how the > random > number distributions fare when analysed with some of the non-parametric tests. > > J' > > > > On Wed, Sep 21, 2011 at 12:50:47PM -0400, Jason Stover wrote: > On Wed, Sep 21, 2011 at 10:12:39AM -0400, Andy Choens wrote: > > If I set a set to a consitent value, say 123, and create a sample in > PSPP will it match the sample created by SPSS? > > I doubt it, but can't be sure becaus the source code to SPSS is kept > secret. > > > If someone > > knows which pseudo number generator is being used by PSPP and SPSS > respectively that would also be a big help so I could > > replicate / confirm output indpenedently. > > As of about 10 years ago, most of SPSS used a multiplicative > congruential random number generator. It had a period of either 2^31 - > 1 or 2^32 - 1. They may still use such a generator, since changing it > would cause users' old syntax to give different answers. > > PSPP uses the Mersenne Twister, which has a period of 2^19937 - 1, as > implemented in GSL. You can see the code for it in src/math/random.c. > > -Jason > > _______________________________________________ > Pspp-users mailing list > Pspp-users@gnu.org > https://lists.gnu.org/mailman/listinfo/pspp-users > > _______________________________________________ Pspp-users mailing list Pspp-users@gnu.org https://lists.gnu.org/mailman/listinfo/pspp-users
