The Colless index is calculated as the sum of \(abs(L - R)\) over all nodes, where L (or R) is the number of extant tips associated with the L (or R) daughter branch at that node. Higher values indicate higher imbalance. Two normalizations are available, where a correction is made for tree size, under either a yule expectation, or a pda expectation.

## References

Colless D H. 1982. Review of: Phylogenetics: The Theory and Practice of Phylogenetic Systematics. Systematic Zoology 31:100-104.

## Examples

```
simulated_tree <- ape::rphylo(n = 10, birth = 1, death = 0)
balanced_tree <- treestats::create_fully_balanced_tree(simulated_tree)
unbalanced_tree <- treestats::create_fully_unbalanced_tree(simulated_tree)
colless(balanced_tree)
#> [1] 4
colless(unbalanced_tree) # should be higher
#> [1] 36
```