On 04-Jun-05 Martin Maechler wrote: >>>>>> "UweL" == Uwe Ligges <[EMAIL PROTECTED]> >>>>>> on Sat, 04 Jun 2005 11:43:34 +0200 writes: > > UweL> (Ted Harding) wrote: > >> On 03-Jun-05 Ted Harding wrote: > >> [...] > >> I have a suggestion (maybe it should also go to R-devel). > >> > >> There are many functions in R whose designated purpose is > >> to return the value of a probability (or a probability > >> density). This designated purpose is in the mind of the > >> person who has coded the function, and is implicit in its > >> usage. > >> > >> Therefore I suggest that every such function should have > >> a built-in internal check that no probability should be > >> less than 0 (and if the primary computation yields such > >> a value then the function should set it exactly to zero), > >> and should not exceed 1 (in which case the function should > >> set it exactly to 1). [And, in view of recent echanges, > >> I would suggest exactly +0, not -0!] > >> > >> Similar for any attempts to return a negative probability > >> density; while of course a positive value can be allowed > >> to be anything. > >> > >> All probabilities would then be guaranteed to be "clean" > >> and issues like the Fisher exact test above would no longer > >> be even a tiny problem. > >> > >> Implementing this in the possibly many cases where it is > >> not already present is no doubt a long-term (and tedious) > >> project. > >> > >> Meanwhile, people who encounter problems due to its absence > >> can carry out their own checks and adjustments! > > UweL> [moved to R-devel] > > UweL> Ted, my (naive?) objection: > UweL> Many errors in the underlying code have been detected by a > function > UweL> returning a nonsensical value, but if the probability is > silently set to > UweL> 0 or 1 ....... > UweL> Hence I would agree to do so in special cases where it makes > sense > UweL> because of numerical issues, but please not globally. > > I agree very much with Uwe's point. > > Further to fisher.test(): This whole thread is > re-hashing a pretty recent bug report on fisher.test() > { "negative p-values from fisher's test (PR#7801)", April '05} > I think that only *because* of the obviously wrong P-values have > we found and confirmed that the refereed and published code > underlying fisher.test() is bogous. Such knowledge would have > been harder to gain if the P-values would have been cut into [0,1]. > > Martin Maechler
Uwe, Martin: Those are very pertinent comments, and they lead me to agree with you! I really had in mind the case where the code for a function was not in question, but the usual "rounding errors" could generate an "impossible" result. So I think that perhaps the best solution is the one I proposed as "interim", namely that people carry out their own checks and adjustments. Best wishes, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <[EMAIL PROTECTED]> Fax-to-email: +44 (0)870 094 0861 Date: 05-Jun-05 Time: 11:40:21 ------------------------------ XFMail ------------------------------ ______________________________________________ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel