[SciPy-User] zscore axis functionality is borked

[Warren Weckesser]

On Wed, Nov 30, 2011 at 2:45 PM, <josef.pktd at gmail.com> wrote:

> On Wed, Nov 30, 2011 at 3:25 PM, Alacast <alacast at gmail.com> wrote:
> > axis=0 (the default) works fine. axis=1, etc, is clearly wrong. Am I
> > misunderstanding how to use this, or is this a bug?
> >
> > In [16]: i = rand(4,4)
> >
> > In [17]: i
> > Out[17]:
> > array([[ 0.85367762,  0.25348857,  0.23572615,  0.50403358],
> >        [ 0.70199066,  0.81872151,  0.47357357,  0.20425537],
> >        [ 0.31042673,  0.25837984,  0.73550134,  0.57970176],
> >        [ 0.42828877,  0.60988596,  0.04059321,  0.73944219]])
> >
> > In [18]: zscore(i, axis=0)
> > Out[18]:
> > array([[ 1.30128758, -0.96195723, -0.52119142, -0.01453907],
> >        [ 0.59653471,  1.38544585,  0.39284654, -1.55756529],
> >        [-1.22271057, -0.94164388,  1.39942427,  0.37494213],
> >        [-0.67511172,  0.51815526, -1.27107939,  1.19716222]])
> >
> > In [19]: zscore(i[:,0])
> > Out[19]: array([ 1.30128758,  0.59653471, -1.22271057, -0.67511172])
> >
> > In [20]: zscore(i[:,0])==zscore(i,axis=0)[:,0]
> > Out[20]: array([ True,  True,  True,  True], dtype=bool)
> >
> > In [21]: zscore(i, axis=1)
> > Out[21]:
> > array([[-0.99378502, -1.59397407, -1.61173649, -1.34342906],
> >        [-1.6379836 , -1.52125275, -1.86640069, -2.13571889],
> >        [-2.09968257, -2.15172946, -1.67460796, -1.83040754],
> >        [-1.29796925, -1.11637205, -1.68566481, -0.98681582]])
> > #The above is obviously wrong, as everything has a negative z score
> >
> > In [22]: zscore(i[0,:])
> > Out[22]: array([ 1.56824016, -0.83321371, -0.90428403,  0.16925757])
> >
> > In [23]: zscore(i[0,:])==zscore(i,axis=1)[0,:]
> > Out[23]: array([False, False, False, False], dtype=bool)
> > #Using axis=1 produces different results from taking a row directly.
> >
> > In [24]: zscore(i, axis=-1)
> > Out[24]:
> > array([[-0.99378502, -1.59397407, -1.61173649, -1.34342906],
> >        [-1.6379836 , -1.52125275, -1.86640069, -2.13571889],
> >        [-2.09968257, -2.15172946, -1.67460796, -1.83040754],
> >        [-1.29796925, -1.11637205, -1.68566481, -0.98681582]])
> > #Getting rows by using axis=-1 is no better (this is the same result as
> > axis=1
>
> This looks like a serious bug to me. I don't know what happened here (.
>
> The docstring example also has negative numbers only.
>
> ???
>
> I'm looking into it
>
>

This is a bug in zscore.  There is a misplaced parenthesis in the code.
This

        return ((a - np.expand_dims(mns, axis=axis) /
                 np.expand_dims(sstd,axis=axis)))

should be this

        return ((a - np.expand_dims(mns, axis=axis)) /
                 np.expand_dims(sstd,axis=axis))


Warren



> Thanks for reporting
>
> Josef
>
>
> >
> >
> >
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> >
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