How is a coincidence matrix constructed for computing Krippendorff's alpha?
The formula in the Wikipedia article seems to be limited to two coders. Look at the sum of values in u for m coders, i I like j, so each value in u is to be matched with one other value in u.  Actually, this sum would pair values of i with values of j and values in j with values i. So, o sub aa would be 2 not 1.  And o sub bb would be 4, not two. 

However, I don’t see how Wikipedia’s formulation could yield coincidence matrices for more than two coders as this would require two sums which pair each of m valors with each of m-1 other values.