I ran a very basic test. I created a .sav file with a series of numbers ranging
from 1-250, in increments of 1. I can provide the file off-list if you'd like,
but its not hard to make.
I then fed this file to both SPSS 11 and PSPP 0.6.2 (current version in Ubuntu
repos). I ran nearly the exct same syntax in both. My syntax looked like;
=====
set seed = 123456789123456789 .
sample .1 .
list .
show seed .
======
The only difference is that in SPSS, I force it to use the Mersenne Twister rng
for consistency. My output differed greatly. The output in both cases surprised
me, because I expected to get a sample of 25 and neither SPSS nor PSPP gave me
a return of 25.
===============================================
PSPP
===============================================
sam_test
--------
10
20
24
45
49
67
71
74
80
82
91
114
144
186
190
192
219
231
236
248
249
===============================================
SPSS
===============================================
9
12
20
29
32
34
43
60
70
72
79
97
117
126
138
145
146
156
159
171
173
174
175
178
179
180
181
188
199
209
231
246
SEED = 1,929,887,249
PSPP failed to return the seed it used. Instead, it broke up it's returns in a
funny way, but that is a bug for another day. At first blush, the results do
not appear to be reproducible between systems.
My next attempt will be to run a similar test in R to see if it matches either
SPSS or PSPP or neither one.
--andy
On Wednesday, September 21, 2011 06:29:01 PM you 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
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