Kendall tau is a correlation coefficient measuring the agreement between the orderings of paired observations 
. In the absence of ties, a pair of observations 
is concordant when 
and discordant when this product is
negative. If 
and 
are the respective numbers of concordant
and discordant pairs among 
observations, then
 |
The coefficient lies between 
and 1. The values 1 and 
correspond to complete agreement and complete reversal of
the two orderings, respectively. Variants of the denominator
account for ties (Kendall 1938).
