Metaconcepts of rooted tree balance

TL;DR

This paper introduces the concept of metaconcepts of tree balance, unifying and generalizing existing indices to better understand the shape characteristics of rooted trees across various research fields.

Contribution

It formalizes metaconcepts as functions depending on shape features, enabling systematic analysis of entire families of tree balance indices beyond individual measures.

Findings

Metaconcepts encompass existing tree balance indices.

Provides a framework for analyzing tree shape characteristics.

Enables systematic study of tree balance measures.

Abstract

Measures of tree balance play an important role in many different research areas such as mathematical phylogenetics or theoretical computer science. Typically, tree balance is quantified by a single number which is assigned to the tree by a balance or imbalance index, of which several exist in the literature. Most of these indices are based on structural aspects of tree shape, such as clade sizes or leaf depths. For instance, indices like the Sackin index, total cophenetic index, and $\widehat{s}$-shape statistic all quantify tree balance through clade sizes, albeit with different definitions and properties. In this paper, we formalize the idea that many tree (im)balance indices are functions of similar underlying tree shape characteristics by introducing metaconcepts of tree balance. A metaconcept is a function $\Phi_f$ that depends on a function $f$ capturing some aspect of tree shape, such as balance values, clade sizes, or leaf depths. These metaconcepts encompass existing indices but also provide new means of measuring tree balance. The versatility and generality of metaconcepts allow for the systematic study of entire families of (im)balance indices, providing deeper insights that extend beyond index-by-index analysis.