[Scipy-tickets] [SciPy] #865: Kendalltau divides by zero

SciPy scipy-tickets@scipy....
Wed Feb 4 03:06:49 CST 2009

```#865: Kendalltau divides by zero
-------------------------+--------------------------------------------------
Reporter:  icmurray     |       Owner:  somebody
Type:  defect       |      Status:  new
Priority:  normal       |   Milestone:  StatisticsCleanup
Component:  scipy.stats  |     Version:  devel
Severity:  normal       |    Keywords:  kendalltau cleanup stats divide by zero nan
-------------------------+--------------------------------------------------
It may be that I have not understood the definition of the kendall-tau
properly, but I think there is a problem with the kendalltau when one or
both input lists ranks all elements as being equal.  In particular:

{{{
>>> from scipy.stats import kendalltau
>>> kendalltau([1,1,1],[3,3,3])
(nan, nan)
>>> kendalltau([1,1,1],[3,3,0])
(nan, nan)
}}}

Now, according to the wikipedia entry for kendall tau:
http://en.wikipedia.org/wiki/Kendall%27s_tau

I'd expect:
{{{
>>> kendalltau([1,1,1],[3,3,3])[0] = 0
}}}
Since the number of concordant pairs is 6, the number of discordant pairs
is 6, and the total number of possible pairs is 6. i.e. (6 - 6)/6 = 0

--
Ticket URL: <http://scipy.org/scipy/scipy/ticket/865>
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```