[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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