(Ted Harding) wrote:
On 03-Jun-05 Ted Harding wrote:
And on mine
(A: PII, Red Had 9, R-1.8.0):
ff <- c(0,10,250,5000); dim(ff) <- c(2,2);
1-fisher.test(ff)$p.value
[1] 1.268219e-11
(B: PIII, SuSE 7.2, R-2.1.0beta):
ff <- c(0,10,250,5000); dim(ff) <- c(2,2);
1-fisher.test(ff)$p.value
[1] -1.384892e-12
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!
[moved to R-devel]
Ted, my (naive?) objection:
Many errors in the underlying code have been detected by a function
returning a nonsensical value, but if the probability is silently set to
0 or 1 .......
Hence I would agree to do so in special cases where it makes sense
because of numerical issues, but please not globally.
Uwe Ligges
Best wishes to all,
Ted.
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E-Mail: (Ted Harding) <[EMAIL PROTECTED]>
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Date: 04-Jun-05 Time: 00:02:32
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