---
title: "Methodological Notes and Limitations"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Methodological Notes and Limitations}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(collapse = TRUE, comment = "#>")
library(taxodist)
```

## Scope

`taxodist` represents each taxon as an ordered lineage from the root of The
Taxonomicon classification to the queried node. The resulting distance is a
property of that classification hierarchy. It is not an estimate of divergence
time, genetic distance, or phylogenetic branch length.

## Definition

Let \(L_A\) and \(L_B\) be two ordered lineages. Let \(h(A,B)\) be the length of
their continuous common prefix, equivalent to the depth of their most recent
common ancestor (MRCA). The distance is

\[
d(A,B) =
\begin{cases}
0, & L_A = L_B, \\
1/h(A,B), & L_A \ne L_B.
\end{cases}
\]

If two lineages have no shared root, the package returns an infinite distance.
Names repeated after the first divergence are not considered evidence of shared
ancestry.

Zero is reserved for the same hierarchy node. Consequently, an ancestor and
one of its descendants have positive distance even though they are connected by
ancestry. Membership and distance answer different questions: `is_member()` and
`taxo_path()` should be used for containment and ancestry queries.

## Ultrametric property

For lineages belonging to one connected hierarchy, the distance is
non-negative, symmetric, and satisfies identity of indiscernibles. It also
satisfies the strong triangle inequality

\[
d(A,C) \leq \max\{d(A,B), d(B,C)\}.
\]

The common prefix of \(A\) and \(C\) must be at least as long as the shorter of
the common prefixes of \((A,B)\) and \((B,C)\). Therefore,

\[
h(A,C) \geq \min\{h(A,B),h(B,C)\}.
\]

Taking reciprocals gives the strong triangle inequality.

The property can also be checked on the offline reference matrix distributed
with the package.

```{r ultrametric-check}
m <- as.matrix(taxobase$statistical_matrix)
tolerance <- sqrt(.Machine$double.eps)

ultrametric_ok <- function(index) {
  i <- index[1]
  j <- index[2]
  k <- index[3]

  d_ij <- m[i, j]
  d_ik <- m[i, k]
  d_jk <- m[j, k]

  d_ij <= max(d_ik, d_jk) + tolerance &&
    d_ik <= max(d_ij, d_jk) + tolerance &&
    d_jk <= max(d_ij, d_ik) + tolerance
}

all(combn(seq_len(nrow(m)), 3, FUN = ultrametric_ok))
```

## Dependence on the source classification

The numerical value depends on the number and arrangement of nodes returned by
The Taxonomicon. Taxonomic revisions, alternative classifications, and
differences in lineage resolution can change both MRCA depth and distance.

Comparisons should therefore use lineages obtained from the same source and,
where possible, the same package and data-retrieval version.

## Ambiguous and missing taxa

Some names correspond to more than one valid biological entry. A numeric
Taxonomicon identifier can be supplied when a specific taxonomic concept is
required.

Missing lineages produce missing pairwise values. Lineages without a common
root produce infinite values. These cases should be resolved before applying
methods that require a complete finite matrix, including clustering and
ordination.

## Interpretation

Smaller values indicate a deeper shared node in the source classification.
They should not be interpreted as:

- elapsed evolutionary time;
- genetic or morphological divergence;
- phylogenetic branch length;
- probability of common ancestry.

Hierarchical clustering of these distances produces a taxonomic similarity
dendrogram, not an independently inferred phylogenetic tree.

## Recommended reporting

Analyses using `taxodist` should report:

- the package version;
- the lineage retrieval date;
- The Taxonomicon as the source classification;
- the distance definition;
- decisions involving ambiguous or missing taxa;
- any clustering, ordination, or permutation method subsequently applied.

Formatted citations for the package and The Taxonomicon are available through
`citation("taxodist")`.
