[Rd] bug with rpois (PR#9106)
rdpeng at gmail.com
rdpeng at gmail.com
Thu Jul 27 19:53:16 CEST 2006
Technically speaking, your version of R is from the previous century. You might
want to check with a more recent version before reporting a bug.
-roger
dolive at math.siu.edu wrote:
> The R poisson random generator rpois appears to have a bug for
> theta 10 or larger. The sample mean of the pseudo variates is too
> small: sample mean approx theta - 0.5.
>
> I use Version 1.1.1 (August 15, 2000) Of R on a Dell OptiPlex computer
> with the
> Windows XP Professional operating system.
>
> Has this bug been fixed in later versions?
>
> (I found another reported rpois bug, but it appears to be different
> and to have been fixed :
>
> rpois gives a large number repeatedly
> kjetikj at astro.uio.no
> Tue, 15 Feb 2000 12:18:28 +0100 (MET))
>
>
>
> The following output helps illustrate the bug.
>
> > zsim
> function(nc = 100, nr = 500, theta = 1)
> {# check rpois function
> x <- matrix(rpois(nr*nc, theta),nrow=nr,ncol=nc)
> z<-apply(x,2,mean)
> list(z=z)}
>
> > zsim(theta=10)
> $z
> [1] 9.594 9.552 9.538 9.448 9.438 9.544 9.630 9.474 9.568 9.700 9.720 9.642
> [13] 9.624 9.328 9.420 9.492 9.368 9.674 9.528 9.436 9.296 9.502 9.260 9.440
> [25] 9.572 9.698 9.372 9.458 9.532 9.570 9.534 9.528 9.572 9.536 9.234 9.416
> [37] 9.400 9.370 9.438 9.628 9.672 9.738 9.454 9.478 9.438 9.496 9.672 9.308
> [49] 9.632 9.386 9.842 9.452 9.632 9.460 9.660 9.688 9.486 9.492 9.576 9.664
> [61] 9.290 9.300 9.304 9.472 9.618 9.480 9.458 9.558 9.782 9.532 9.198 9.258
> [73] 9.384 9.630 9.520 9.646 9.554 9.406 9.390 9.554 9.610 9.382 9.604 9.638
> [85] 9.304 9.586 9.848 9.434 9.460 9.644 9.450 9.542 9.502 9.446 9.828 9.364
> [97] 9.664 9.628 9.704 9.354
>
> Notice that all 100 sample means of size 500 are less than 10, about
> half should be larger.
>
> The following commands also suggest that the rpois function is biased
> for theta > 10.
>
> > hist(zsim(theta=1)$z)
> > hist(zsim(theta=10)$z)
> > hist(zsim(theta=100)$z)
> > hist(zsim(theta=1000)$z)
>
>
> Best Regards,
>
>
> David Olive
> dolive at math.siu.edu
>
> ______________________________________________
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>
--
Roger D. Peng | http://www.biostat.jhsph.edu/~rpeng/
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