Duncan Murdoch <[EMAIL PROTECTED]> writes: > On 3/22/2006 3:52 AM, [EMAIL PROTECTED] wrote: > >>>>>> "cspark" == cspark <[EMAIL PROTECTED]> > >>>>>> 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.
Well, the p-functions are constant on the intervals between integers... (Or, did you refer to the lack of a warning? One point could be that cumulative p.d.f.s extends naturally to non-integers, whereas densities don't really extend, since they are defined with respect to counting measure on the integers.) > 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? Most of these round-near-integer issues were spurred by real programming problems. It is somewhat hard to come up with a problem that leads you generate a binomial variate value with "floating point noise", but I'm quite sure that we'll be reminded if we try to change it... (One potential issue is back-calculation to counts from relative frequencies). -- O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
