function combineVariances(variance1, mean1, n1, variance2, mean2, n2) {
    const newMean = combineMeans(mean1, n1, mean2, n2);

    return (
        (n1 * (variance1 + Math.pow(mean1 - newMean, 2)) +
            n2 * (variance2 + Math.pow(mean2 - newMean, 2))) /
        (n1 + n2)
    );
}
When combining two lists of values for which one already knows the variances,
one does not have to necessary recompute the variance of the combined lists
in linear time. They can instead use this function to compute the combined
variance by providing the variance, mean & number of values of the first list
and the variance, mean & number of values of the second list.

@since 3.0.0
@param {number} variance1 variance of the first list
@param {number} mean1 mean of the first list
@param {number} n1 number of items in the first list
@param {number} variance2 variance of the second list
@param {number} mean2 mean of the second list
@param {number} n2 number of items in the second list
@returns {number} the combined mean

@example
combineVariances(14 / 3, 5, 3, 8 / 3, 4, 3); // => 47 / 12