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http://issues.apache.org/bugzilla/show_bug.cgi?id=31688
Proposal for Poisson distributuion
------- Additional Comments From brent@worden.org 2004-11-02 01:37 -------
Few comments:
1) The createFormat method uses replaceAll, a JDK 1.4 call. I think [math] is
meant to be compatible with JDK 1.3 if not JDK 1.2.
2) I see createFormat is being used to force precision on the inverse
cumulative probs. Is there a reason this is being done on intermediate
results? Is it necessary at all?
3) For the probability method, I would suggest using the ol' Exp[Log[f(x)]]
trick to avoid numerical overflow. n! is a very volatile method for even
moderate n values.
4) Can the parameter be renamed to something a little more descriptive? I
would suggest mean.
5) I think the cumulative probability method can be optimized one of two ways,
or by utilizing both. Cumulative probabilities for Poisson can be expressed in
terms of the regularized gamma function which we already have implemented. The
performance of this approach would not deteriorate for large x as the current
loop does. At a minimum, instead of making calls to probability for each
summation term, the ratio of successive terms should be used to alleviate the
need to call n! for every term.
6) Is there a compelling reason not to use the inverseCumulativeProbability
method provided by the AbstractDiscreteDistribution class?
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