The ordinary least squares estimate of a branch length (b) is given by
where dij is the pairwise distance between sequences i and j. The coefficients wij’s depend on whether the branch under consideration is internal or external.
Coefficients wij’s for an internal branch
where, mA, mB, mC, and mC are the numbers of sequences in clusters A, B, C, and D, respectively.
Coefficients wij’s for an external branch
where, mA and mB are the numbers of sequences in clusters A and B.