On Sat, May 5, 2012 at 5:59 PM, nicky van foreest <vanforeest@gmail.com> wrote:
Hi,
I just looked through the discussion in the ticket. Both sides (1: the scale k should be an int, 2) allow k to be a float) make sense. From my background in queueing I am inclined to say that k should be restricted to the integers as in queueing theory the Erlang-k distribution is used to model some distribution for which k can only be an int. On the other hand, I am unsure whether a user of the Erlang distribution should be protected from filling in a float. In all (?) books on queueing and probability it is written that the Erlang distribution is a special case of the gamma distribution, so users of the Erlang distribution should know this (.... kind of, hopefully).
From an implementation point of view I think that aliasing Erlang to the gamma distribution makes good sense, and I don't believe that the users or the Erlang distribution require to be protected against filling in floats. Perhaps a sentence in the docstring of the Erlang distribution that it is an alias of the gamma distribution, hence does not check on the scale being an int, will prevent some potential misuse.
Thanks for the comments. Do you think it would be useful to have fit() restrict to integers? I guess currently nobody uses erlang.fit() because, at least in the example, it doesn't work with default parameters. import numpy as np from scipy import stats #add fitstart to erlang, otherwise it doesn't work stats.erlang._fitstart = stats.gamma._fitstart np.random.seed(876589) rvs = stats.erlang.rvs(5, size=500) for dist in [stats.erlang, stats.gamma]: print '\n', dist.name p0 = dist.fit(rvs) print stats.gamma.nnlf(p0, rvs), p0 print for k in range(10): p = dist.fit(rvs, f0=k) print dist.nnlf(p, rvs), p Josef
Nicky
On 5 May 2012 22:20, <josef.pktd@gmail.com> wrote:
Should we restrict the shape parameter to be an integer instead of a float?
http://projects.scipy.org/scipy/ticket/1647
Let's ask the users:
Does anyone want an exception if the shape parameter is not an integer? Is there a demand or use case for estimating the shape parameter as an integer instead of a float?
right now erlang and gamma are essentially the same, as far as I can see
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