Hi, I agree. Anything that makes the behavior of the distribution functions more intuitive is helpful, at least to me. BTW, I find the term loc already by itself very confusing---what does it actually mean? For instance,
Help on gamma_gen in module scipy.stats.distributions object ...
| cdf(self, x, *args, **kwds) | Cumulative distribution function at x of the given RV. | | Parameters | ---------- | x : array-like | quantiles | arg1, arg2, arg3,... : array-like | The shape parameter(s) for the distribution (see docstring of the | instance object for more information) | loc : array-like, optional | location parameter (default=0) | scale : array-like, optional | scale parameter (default=1) I am inclined to characterize the gamma distbution by means of n (number of stages if one is used to the Erlang distribution) and the rate parameter lambda, say, and I am clueless as to the meaning of scale and location here. Actually, I am not alone in this: see for instance: http://www.johndcook.com/blog/2009/07/20/probability-distributions-scipy/ Of course, this is not to say that I am not happy with the distribution package. It makes me a happier man every day :-) Nicky 2009/8/6 Pierre GM <pgmdevlist@gmail.com>:
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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