The mean I value is defined for all nodes with at least 4 tips connected, such that different topologies can be formed. Then, for each node, I = (nm - nt/2) / (nt - 1 - nt/2), where nt is the total number of tips descending from that node, nm is the daughter branch leading to most tips, and nt/2 is the minimum size of the maximum branch, rounded up. Following Purvis et al 2002, we perform a correction on I, where we correct I for odd nt, such that I' = I * (nt - 1) / nt. This correction ensures that I is independent of nt. We report the mean value across all I' (again, following Purvis et al. 2002).
References
G. Fusco and Q. C. Cronk. A new method for evaluating the shape of large phylogenies. Journal of Theoretical Biology, 1995. doi: 10.1006/jtbi.1995.0136. A. Purvis, A. Katzourakis, and P.-M. Agapow. Evaluating Phylogenetic Tree Shape: Two Modifications to Fusco & Cronks Method. Journal of Theoretical Biology, 2002. doi: 10.1006/jtbi.2001.2443.