paired
The statistics for the pair of data names given by the strings. If the data names don't exist then an exception will occur. The name of the statistic is formed as "$s1 - $s2". If that difference does not exist, then null is returned.
Return
a Statistic collected over the paired difference of "$s1 - $s2"
Parameters
s1
the name of data set number 1
s2
the name of data set number 2