[Rd] Problem in random number generation for Marsaglia-Multicarry + Kinderman-Ramage
pd@|gd @end|ng |rom gm@||@com
Thu Aug 12 15:07:52 CEST 2021
With these matters, one has to be careful to distinguish between method error and implementation error.
The reason for changing the RNG setup in R v. 1.7.0 was pretty much this kind of unfortunate interaction between M-M and K-R. There are even more egregious examples for the distribution of maxima of normal variables. Try e.g.
RNGversion("1.6.0") # Marsaglia-Multicarry, Kinderman-Ramage
s <- replicate(1e6,max(rnorm(10)))
(A further bug in K-R was fixed in 1.7.1, but that is tangential to this.)
A glimpse of the source of the problem is seen in the "microcorrelations" in this:
RNGkind("Mar");m <- matrix(runif(4e7),2)
m <- matrix(runif(4e7),2)
These examples are from 2003, so the issue has been known for almost 2 decades. However, to the best of our knowledge, the M-M RNG is a faithful implementation of their method, so we have left the RNG in R's arsenal, in case someone needed it for some specific purpose.
> On 12 Aug 2021, at 11:51 , GILLIBERT, Andre <Andre.Gillibert using chu-rouen.fr> wrote:
> Dear R developers,
> In my opinion, I discovered a severe flaw that occur with the combination of the Marsaglia-Multicarry pseudo-random number generator associated to the Kinderman-Ramage algorithm to generate normally distributed numbers.
> The sample program is very simple (tested on R-4.1.1 x86_64 on Windows 10):
> set.seed(1, "Marsaglia-Multicarry", normal.kind="Kinderman-Ramage")
> poisson.test(sum(v < (-4)))$conf.int # returns c(34.5, 62.5)
> poisson.test(sum(v > (4)))$conf.int # returns c(334.2, 410.7)
> pnorm(-4)*1e7 # returns 316.7
> There should be approximatively 316 values less than -4 and 316 values greater than +4, bug there are far too few values less than -4.
> Results are similar with other random seeds, and things are even more obvious with larger sample sizes.
> The Kinderman-Ramage algorithm is fine when combined to Mersenne-Twister, and Marsaglia-Multicarry is fine when combined with the normal.kind="Inversion" algorithm, but the combination of Marsaglia-Multicarry and Kinderman-Ramage seems to have severe flaws.
> R should at least warn for that combination !
> What do you think? Should I file a bug report?
> Andr� GILLIBERT
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