variance Of Difference
The variance for the pair of data names given by the strings. If the data names don't exist or the difference "$s1 - $s2" or "$s2 - $s1" does not exist then an exception will occur. The variance of the differences are symmetric. That is variance of "$s1 - $s2" is equal to the variance of "$s2 - $s1".
Return
variance for the pair of data names given by the strings "$s1 - $s2" or "$s2 - $s1"
Parameters
s1
the name of data set number 1
s2
the name of data set number 2