[R] Strange means of numbers drawn from rpois

[Kjetil Kjernsmo]

Dear all,

I think I must be going crazy.... If I do
> mean(rpois(1000000, 14))
I get:
[1] 13.50420
and again:
> mean(rpois(1000000, 14))
[1] 13.49896
> mean(rpois(1000000, 14))
[1] 13.50161
> mean(rpois(1000000, 15))
[1] 14.49250
> mean(rpois(1000000, 15))
[1] 14.49897
> mean(rpois(1000000, 14.5))
[1] 13.99689
> mean(rpois(1000000, 14.5))
[1] 13.9963

This was on a
platform alphaev6-dec-osf4.0e
major    1
minor    1.1
checking out on a
platform sparc-sun-solaris2.7
major    1
minor    1.0
I get the same
> mean(rpois(100000, 14.5))
[1] 14.00347
> mean(rpois(100000, 14.5))
[1] 14.00667
> mean(rpois(100000, 14.5))
[1] 14.00675

It's fishy, isn't it...? I have generated lots of numbers, but I haven't
seen anything like this before....

I have also done

> sapply(1:15, function(n) mean(rpois(1000000, n)))
 [1]  1.000052  1.999150  2.996975  4.003843  4.996136  5.999065  6.999725
 [8]  7.998869  9.003127  9.518397 10.512347 11.500405 12.510494 13.499788
[15] 14.504661

So, something happens at 10... And again:

> sapply(1:15, function(n) mean(rpois(1000000, n)))
 [1]  1.000173  2.001639  2.997016  4.000163  5.000802  5.992751  6.999794
 [8]  7.997315  8.998479  9.517824 10.508328 11.507336 12.501885 13.502146
[15] 14.496810

Starting with --vanilla gives pretty much the same result...

trying different RNGs:
> RNGkind("Mersenne")
> sapply(1:15, function(n) mean(rpois(1000000, n)))
 [1]  1.000563  1.998802  2.998526  3.997537  4.999230  5.996591  6.999596
 [8]  7.998982  9.000783  9.510220 10.509562 11.505009 12.505001 13.504522
[15] 14.500956
> RNGkind("Knut")
> sapply(1:15, function(n) mean(rpois(1000000, n)))
 [1]  0.999031  2.000444  3.002296  3.997795  4.998880  5.995267  7.001894
 [8]  7.999983  9.000267  9.513537 10.507142 11.502634 12.498944 13.498615
[15] 14.502410

After printing rpois (to see that I haven't replaced it with something
strange) and version, I call
> plot(sapply(1:15, function(n) mean(rpois(1000000, n))), t='l')
getting a plot that isn't a straight line, and then:
> sapply(1:15, function(n) mean(rpois(1000000, n)))
 [1]  0.997961  1.999509  3.000748  3.999825  5.002983  6.004424  6.998825
 [8]  7.997999  8.998341  9.766942 10.781751 11.817181 12.856154 13.900369
[15] 14.999378
and again:
> sapply(1:15, function(n) mean(rpois(1000000, n)))
 [1]  0.998570  2.000155  2.999534  4.002987  4.997999  5.996326  6.996761
 [8]  7.996728  8.997265  9.764972 10.793913 11.822874 12.859886 13.903846
[15] 15.000311

Weird....?

Best,

Kjetil
-- 
Kjetil Kjernsmo
Graduate astronomy-student                    Problems worthy of attack
University of Oslo, Norway            Prove their worth by hitting back
E-mail: kjetikj at astro.uio.no                                - Piet Hein
Homepage <URL:http://www.astro.uio.no/~kjetikj/>
Webmaster at skepsis.no


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