[Numpy-discussion] numpy histogram normed=True (bug / confusing behavior)

Nils Becker n.becker@amolf...
Fri Aug 6 15:53:58 CDT 2010


Hi again,

first a correction: I posted

> I believe np.histogram(data, bins, normed=True) effectively does :
>>> np.histogram(data, bins, normed=False) / (bins[-1] - bins[0]).
>>>
>>> However, it _should_ do
>>> np.histogram(data, bins, normed=False) / bins_widths

but there is a normalization missing; it should read

I believe np.histogram(data, bins, normed=True) effectively does
np.histogram(data, bins, normed=False) / (bins[-1] - bins[0]) / data.sum()

However, it _should_ do
np.histogram(data, bins, normed=False) / bins_widths / data.sum()

Bruce Southey replied:
> As I recall, there as issues with this aspect.
> Please search the discussion regarding histogram especially David
> Huard's reply in this thread:
> http://thread.gmane.org/gmane.comp.python.numeric.general/22445
I think this discussion pertains to a switch in calling conventions
which happened at the time. The last reply of D. Huard (to me) seems to
say that they did not fix anything in the _old_ semantics, but that the
new semantics is expected to work properly.

I tried with an infinite bin:
counts, dmy = np.histogram([1,2,3,4], [0.5,1.5,np.inf])
counts
array([1,3])
ncounts, dmy = np.histogram([1,2,3,4], [0.5,1.5,np.inf], normed=1)
ncounts
array([0.,0.])

this also does not make a lot of sense to me. A better result would be
array([0.25, 0.]), since 25% of the points fall in the first bin; 75%
fall in the second but are spread out over an infinite interval, giving
0. This is what my second proposal would give. I cannot find anything
wrong with it so far...

Cheers, Nils


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