[SciPy-User] Rician distributions lacks sigma parameter

Warren Weckesser warren.weckesser@enthought....
Wed Apr 4 16:00:06 CDT 2012


On Wed, Apr 4, 2012 at 12:59 PM, <josef.pktd@gmail.com> wrote:

> On Wed, Apr 4, 2012 at 1:53 PM,  <josef.pktd@gmail.com> wrote:
> > On Wed, Apr 4, 2012 at 1:33 PM, Morten Kjeldgaard <mok@bioxray.dk>
> wrote:
> >> Thanks for replies Josef and Warren!
> >>
> >> I think my current limitation is that I don't fully grasp how the
> >> shape and scale parameters are propagated to the individual stats
> >> distributions, and alas the autogenerated documentation isn't always
> >> very helpful.
> >>
> >>> Given the parameters nu and sigma (as shown in the wikipedia
> >>> article), you use scipy.stats.rice by setting the scale=sigma and
> >>> the shape parameter b=nu/sigma.  You can use the following script to
> >>> verify this:
> >>
> >> Like you write, the script works fine with parameters (nu, sigma) =
> >> (3.45, 0.35), but it actually fails when I try to reproduce the plots
> >> in the wikipedia article. When setting (nu, sigma) = (0, 1), I get the
> >> following:
> >
> > the shape parameter nu has to be strictly positive, eg. nu=1e-10 works
> > there is a problem with the calculation for nu equal to zero
>
> But the _pdf doesn't have a problem
> >>> stats.rice._pdf(np.linspace(0,4,11),0.)
> array([ 0.        ,  0.36924654,  0.58091923,  0.58410271,  0.44485968,
>        0.27067057,  0.13472343,  0.05555507,  0.01912327,  0.00552172,
>        0.00134185])
>
> >>> stats.rice.pdf(np.linspace(0,4,11),0.)
> array([ nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan])
> >>>
>
> so it should be possible to fix it for the nu=0 case, (define a>= 0 ?)
>
> Josef
>
>


I created a ticket for this: http://projects.scipy.org/scipy/ticket/1639

Warren




>
> >
> > Josef
> >
> >>
> >> AssertionError:
> >> Not equal to tolerance rtol=1e-07, atol=0
> >>
> >> x and y nan location mismatch:
> >>  x: array([  0.00000000e+00,   3.99680128e-02,   7.97444092e-02,
> >>          1.19139103e-01,   1.57965051e-01,   1.96039735e-01,
> >>          2.33186584e-01,   2.69236346e-01,   3.04028363e-01,...
> >>  y: array([ nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,
> >> nan,  nan,
> >>         nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,
> >> nan,
> >>         nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,  nan,
> >> nan,...
> >>
> >> In other words, your defined function rice_pdf works, but stats.rice
> >> does not.
> >>
> >> Cheers,
> >> Morten
> >>
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