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Table 5 Definitions for the tree statistics

From: Employing phylogenetic tree shape statistics to resolve the underlying host population structure

Tree statistics

Definition

References

Cherry

Pair of leaves that is adjacent to a common ancestor node.

[18, 29, 32]

Normalized number of cherries

Number of cherries divided by half the number of tips in a tree.

[46]

Sackin index

Sum of all the number of edges from a leaf to a root for each of a leaf in a tree.

[18, 24]

Normalized Sackin index

Sackin index divided by \((0.5\times (n(n+1))-1)\), where n is the number of tips.

[46]

Colless index

Sum of absolute differences between left and right hand leaves (terminal tips) subtended at each internal node of a tree, the root inclusive.

[24, 45]

Normalized Colless index

Colless index divided by \(\frac{(n-1)(n-2)}{2}\), where n is the number of tips.

[45]

Total cophenetic index

Sum of all depths of the lowest common ancestor for all pairs of leaves in a tree.

[33]

Ladder length

Ratio of maximum number of connected internal nodes with a single descendant leaf to number of leaves in a tree.

[25]

Maximum depth

Maximum number of edges from a leaf to a root for all the leaves in a tree.

[25]

Maximum width

Maximum number of nodes for each possible depth of a tree.

[25]

Width-depth ratio

Ratio of maximum width of a tree to its maximum depth.

[25]