On Thu, Aug 6, 2009 at 16:21, nicky van foreest<vanforeest@gmail.com> wrote:
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.
Every probability distribution can be generalized to accept a location and scale parameter even if their standard treatments do not. pdf(x; loc,scale) -> pdf((x-loc)/scale)/scale The other related functions transform in easily derivable ways. This is covered at the top of the document scipy/stats/continuous.lyx in the source distribution. The reason we do this is partly generality and mostly convenience of implementation; all of the distributions can share the shifting and scaling code instead of implementing it separately. I once floated the idea of removing this for the distributions whose standard definitions do not include such parameters, specifically gamma. However, there was an objection from someone who apparently has used a "shifted gamma" distribution to model sunspot radii where loc>0, if I remember correctly, so I dropped my proposal. If you don't need to use them, don't. If you want to prevent confusion, help port the LyX documentation into the main documentation. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco