[Rd] chi-squared with zero df (PR#10551)
maechler at stat.math.ethz.ch
maechler at stat.math.ethz.ch
Mon Jan 7 18:20:43 CET 2008
>>>>> "MM" == Martin Maechler <maechler at stat.math.ethz.ch>
>>>>> on Mon, 7 Jan 2008 09:50:15 +0100 (CET) writes:
>>>>> "JL" == Jerry Lewis <Jerry.Lewis at biogenidec.com>
>>>>> on Mon, 7 Jan 2008 05:20:23 +0100 (CET) writes:
JL> Full_Name: Jerry W. Lewis
JL> Version: 2.6.1
JL> OS: Windows XP Professional
JL> Submission from: (NULL) (24.147.191.250)
JL> pchisq(0,0,ncp=lambda) returns 0 instead of exp(-lambda/2)
MM> Yes. As we know, chi-square(df=0, ncp=lambda) has a point mass of
MM> exp(-lambda/2) at 0.
MM> Hence pchisq() has a corresponding jump there; strictly, it's a
MM> matter of definition (left- / right- continuity, etc) what
MM> pchisq() should be there, but indeed, the usual definition --
MM> which we also follow for the discrete distributions --
MM> is "cadlag" (continue à droite, limite à gauche),
MM> and you are right.
MM> That's easily fixed.
JL> pchisq(x,0,ncp=lambda) returns NaN
JL> instead of exp(-lambda/2)*(1 + SUM_{r=0}^infty
JL> ((lambda/2)^r / r!) pchisq(x, df + 2r))
MM> Not on my Linux computers; e.g.,
>> pchisq(0:10, 0,1)
MM> [1] 0.0000000 0.7328798 0.8193100 0.8781745 0.9181077 0.9451020 0.9632911
MM> [8] 0.9755110 0.9836985 0.9891706 0.9928194
MM> but I can see the problem on Windows (Server 2003 R2, x64 edition),
MM> where I get
>> pchisq(0:10, 0,1)
MM> [1] 0 NaN NaN ... NaN
MM> aah, and I also see it on a 32-bit Linux computer.
and actually did see it on all computers; I was wrong above.
It's been a consequence of an "improvement" introduced for R 2.3.0;
unfortunately that made the (df=0, ncp < 80) wrongly return NaN.
Now fixed in both R-patched and R-devel.
Thank you once more for the bug report!
Martin
JL> qchisq(.7,0,ncp=1) returns 1.712252 instead of 0.701297103
MM> On my 64-bit Linux computer I get the correct result where as
MM> the above Windows and our 32-bit Linux give *different* (but
MM> wrong) results.
JL> qchisq(exp(-1/2),0,ncp=1) returns 1.238938 instead of 0
MM> Not on my 64-bit Linux where e.g.,
>> lam <- 1:10; qchisq(exp(-lam), 0, ncp=lam)
MM> [1] 2.225074e-308 2.225074e-308 2.225074e-308 2.225074e-308 2.225074e-308
MM> [6] 2.225074e-308 2.225074e-308 2.225074e-308 2.225074e-308 2.225074e-308
MM> i.e. "numerically 0"
MM> {Note that I've known about problems with our non-central chisq
MM> algorithms, but these were all of very extreme cases...}
MM> In summary, we seem to have an issue with our algorithms failing
MM> on some platforms.
MM> I'll investigate.
MM> Martin Maechler, ETH Zurich
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