[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:

 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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