[Rd] pbinom( ) function (PR#8700)
Duncan Murdoch
murdoch at stats.uwo.ca
Wed Mar 22 13:40:11 CET 2006
On 3/22/2006 3:52 AM, maechler at stat.math.ethz.ch wrote:
>>>>>> "cspark" == cspark <cspark at clemson.edu>
>>>>>> on Wed, 22 Mar 2006 05:52:13 +0100 (CET) writes:
>
> cspark> Full_Name: Chanseok Park Version: R 2.2.1 OS: RedHat
> cspark> EL4 Submission from: (NULL) (130.127.112.89)
>
>
>
> cspark> pbinom(any negative value, size, prob) should be
> cspark> zero. But I got the following results. I mean, if
> cspark> a negative value is close to zero, then pbinom()
> cspark> calculate pbinom(0, size, prob).
>
> >> pbinom( -2.220446e-22, 3,.1)
> [1] 0.729
> >> pbinom( -2.220446e-8, 3,.1)
> [1] 0.729
> >> pbinom( -2.220446e-7, 3,.1)
> [1] 0
>
> Yes, all the [dp]* functions which are discrete with mass on the
> integers only, do *round* their 'x' to integers.
>
> I could well argue that the current behavior is *not* a bug,
> since we do treat "x close to integer" as integer, and hence
> pbinom(eps, size, prob) with eps "very close to 0" should give
> pbinom(0, size, prob)
> as it now does.
>
> However, for esthetical reasons,
> I agree that we should test for "< 0" first (and give 0 then) and only
> round otherwise. I'll change this for R-devel (i.e. R 2.3.0 in
> about a month).
>
> cspark> dbinom() also behaves similarly.
>
> yes, similarly, but differently.
> I have changed it (for R-devel) as well, to behave the same as
> others d*() , e.g., dpois(), dnbinom() do.
Martin, your description makes it sound as though dbinom(0.3, size,
prob) would give the same answer as dbinom(0, size, prob), whereas it
actually gives 0 with a warning, as documented in ?dbinom. The d*
functions only round near-integers to integers, where it looks as though
near means within 1E-7. The p* functions round near integers to
integers, and truncate others to the integer below.
I suppose the reason for this behaviour is to protect against rounding
error giving nonsense results; I'm not sure that's a great idea, but if
we do it, should we really be handling 0 differently?
Duncan Murdoch
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