[SciPy-dev] stats.distributions.poisson loc parameter : is it wise ?
Pierre GM
pgmdevlist@gmail....
Thu Aug 6 15:34:03 CDT 2009
All,
Consider the poisson distribution in stats.distributions: it requires
a mandatory argument, `mu`, as the mean/variance of the distribution.
All is fine, but the `loc` parameter is still available, and that's my
problem. When `loc` is not 0, the mean becomes `mu+loc`,
`.cdf(range(loc))==0`, but the variance stays `mu`. That's a bit
confusing.
I thought I could use `loc` as a way to control truncation, but that
doesn't seem to work either: emulating zero-truncation by using
`loc=1` gives a distribution with a mean `mu+1` when is should be `mu/
(1-exp(-mu))` (the exact expression for zero-truncation).
In short, I don't really see any advantage in having a location
parameter for the Poisson distribution. AAMOF, for any discrete
distribution. I suggest we would implement some mechanism to force loc
to 0 while outputting a warning.
Any comment ?
P.
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