Computes the correlation of two variables given summary statistics of the
variables. All of these arguments are summed over the variable:
x' is the sum<br> of the x's, x2' is the sum of the squares of the x's, and
xy' is the sum of<br> the cross-products, which is also known as the inner product of the variables x<br> and y. Of course, n' is the number of data values in each variable.