ewCollessI | R Documentation |

This function calculates the equal weights Colless index `I_2(T)`

for a
given rooted binary tree `T`

. `I_2(T)`

is defined as

`I_2(T)=\frac{1}{n-2}\cdot\sum_{u\in V_{in}(T), n_u>2} \frac{|n_{u_a}-n_{u_b}|}{n_u-2}`

in which `V_{in}(T)`

denotes the set of all inner vertices of `T`

,
and in which `n_u`

, `n_{u_a}`

and `n_{u_b}`

denote the number of
leaves in the pending subtrees that are rooted at `u`

and the two direct
descendants of `u`

. The equal weights Colless index is an imbalance index.

For `n=1`

and `n=2`

the function returns `I_2(T)=0`

and a warning.

For details on the equal weigths Colless index, see
also Chapter 14 in "Tree balance indices: a comprehensive survey" (https://doi.org/10.1007/978-3-031-39800-1_14).

```
ewCollessI(tree)
```

`tree` |
A rooted binary tree in phylo format. |

`ewCollessI`

returns the equal weights Colless index of the given tree.

Luise Kuehn

A. O. Mooers and S. B. Heard. Inferring Evolutionary Process from Phylogenetic Tree Shape. The Quarterly Review of Biology, 72(1), 1997. doi: 10.1086/419657.

```
tree <- ape::read.tree(text="((((,),),(,)),(((,),),(,)));")
ewCollessI(tree)
```

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