Where the summation is used in place of integration since the distribution is discrete. The vector is . Thus, one can write the fundamental variance matrix as , where I is an identity matrix. To estimate this fundamental variability, one would use the matrix . Note that this is a slight abuse of standard mathematical notation, used in order to make the transition to computation easier.
X is and is .
One would add or, equivalently, to the diagonal elements of this matrix in order to calculate the total variability.