This document outlines the similarity measures employed by the generalized Robinson–Foulds distances implemented in this package.

Generalized RF distances are introduced elsewhere; before you read further, you may also wish to revisit how to use the ‘TreeDist’ package, and relevant principles of information theory.

Scoring each pair of splits according to their mutual clustering information (Smith, 2020) (see separate vignette) results in a information-based tree distance metric that recognizes similarity in tree structure even when every possible pairing of splits conflicts:

```
VisualizeMatching(MutualClusteringInfo, AtoJ, swapAJ,
Plot = TreeDistPlot, matchZeros = FALSE, prune = c(5, 18))
```

`## [1] 1.348339`

Because no pair of non-trivial splits has zero mutual clustering
information, even a dissimilar pairing (such as `HI|ABCDEFG`

⇒ `EI|ABCDFGH`

below) is (slightly) preferable to leaving a
split unpaired.

The Nye *et al*. (2006) tree
similarity metric scores pairs by considering the elements held in
common between subsets of each split.

Consider a pair of splits `ABCDEF|GHIJ`

and
`ABCDEIJ|FGH`

. These can be aligned thus:

```
ABCDEF | GHIJ
ABCDE IJ|FGH
```

The first pair of subsets, `ABCDEF`

and
`ABCDEIJ`

, have five elements in common (`ABCDE`

),
and together encompass eight elements (`ABCDEFIJ`

). Their
*subset score* is thus \(\frac{5}{8}\).

The second pair of subsets, `GHIJ`

and `FGH`

,
have two elements (`GH`

) in common, of the five total
(`FGHIJ`

), and hence receive a subset score of \(\frac{2}{5}\).

This split alignment then receives an *alignment score*
corresponding to the lower of the two subset scores, \(\frac{2}{5}\).

We must now consider the other alignment of this pair of splits,

```
ABCDEF | GHIJ
FGH|ABCDE IJ
```

This yields subset scores of \(\frac{1}{8}\) and \(\frac{2}{9}\), and thus has an alignment score of \(\frac{1}{8}\). This alignment gives a lower score than the other, so is disregarded. The pair of splits is allocated a similarity score corresponding to the better alignment: \(\frac{2}{5}\).

As such, splits that match exactly will receive a similarity score of 1, in a manner analogous to the Robinson–Foulds distance. (This is despite the fact that some splits are more likely to match than others.) It is not possible for a pair of splits to receive a zero similarity score.

`## [1] 3.5`

Böcker *et al*. (2013) employ
the same split similarity calculation as Nye *et al.* (above),
which they suggest ought to be raised to an arbitrary exponent in order
to down-weight the contribution of paired splits that are not identical.
In order for the metric to converge to the Robinson–Foulds metric as the
exponent grows towards infinity, the resulting score is then
doubled.

`## [1] 4`

`## [1] 5.638889`

The figure below shows how the JRF distance between the two trees
plotted above varies with the value of the exponent *k*, relative
to the Nye *et al.* and Robinson–Foulds distances between the
trees:

The theoretical and practical performance of the JRF metric, and its
speed of calculation, are best at lower values of *k* (Smith, 2020), raising the question of whether
an exponent is useful.

Böcker *et al*. (2013) suggest
that ‘reasonable’ matchings exhibit a property they term
*arboreality*. Their definition of an arboreal matching supposes
that trees are rooted, in which case each split corresponds to a clade.
In an arboreal matching on a rooted tree, no pairing of splits conflicts
with any other pairing of splits. Consider the case where splits ‘A’ and
‘B’ in tree 1 are paired respectively with splits ‘C’ and ‘D’ in tree 2.
If ‘A’ is paired with ‘C’ and ‘B’ with ‘D’, then to avoid conflict:

if A is nested within B, then C must be nested within D

if B is nested within A, then D must be nested within C

if A and B do not overlap, then C and D must not overlap.

Equivalent statements for unrooted trees are a little harder to express.

Unfortunately, constructing arboreal matchings is NP-complete, making an optimal arboreal matching slow to find. A faster alternative is to prohibit pairings of contradictory splits, though distances generated under such an approach are theoretically less coherent and practically no more effective than those calculated when contradictory splits may be paired, so the only advantage of this approach is a slight increase in calculation speed (Smith, 2020).

Bogdanowicz & Giaro (2012) propose
an alternative distance, which they term the Matching Split Distance.
(This approach was independently proposed by Lin *et al.* (2012).)

`## [1] 5`

Note that the visualization shows the difference, rather than the similarity, between splits.

Similar to the Nye *et al*. similarity metric, this method
compares the subsets implied by a pair of splits. Here, the relevant
quantity is the number of elements that must be moved from one subset to
another in order to make the two splits identical. With the pair of
splits

```
ABCDEF | GHIJ
ABCDE IJ|FGH
```

three leaves (‘F’, ‘I’ and ‘J’) must be moved before the splits are
identical; as such, the pair of splits are assigned a difference score
of three.

Formally, where \(S_i\) splits \(n\) leaves into bipartitions \(A_i\) and \(B_i\), the difference score is calculated
by

\(n - m\)

where \(m\) counts the number of leaves that already match, and is defined as

\(m = \max\{|A_1 \cap A_2| + |B_1 \cap B_2|, |A_1 \cap B_2| + |B_1 \cap A_2|\}\)

```
MatchingSplitDistance(read.tree(text='((a, b, c, d, e, f), (g, h, i, j));'),
read.tree(text='((a, b, c, d, e, i, j), (g, h, f));'))
```

`## [1] 3`

This distance is difficult to normalize, as it is not easy to calculate its maximum value.

In the matching split distance, \(m\) represents a simple count of the number of shared taxa. An alternative is to measure the phylogenetic information content of the largest split consistent with \(S_1\) and \(S_2\):

\(m = \max\{h(A_1 \cap A_2 | B_1 \cap B_2), h(A_1 \cap B_2 | B_1 \cap A_2)\}\)

The most information-rich split consistent with

```
ABCDEF | GHIJ
ABCDE IJ|FGH
```

is `ABCDE | GH`

, which contains

`## [1] 3.169925`

bits of phylogenetic information. This value can be used as a similarity score for this pairing of splits.

`## [1] 17.27586`

Böcker, S., Canzar, S., & Klau, G. W. (2013). The generalized
Robinson-Foulds metric. In A. Darling & J.
Stoye (Eds.), *Algorithms in Bioinformatics.
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Computer Science, vol 8126* (pp. 156–169).
Berlin, Heidelberg: Springer. doi:10.1007/978-3-642-40453-5_13

Bogdanowicz, D., & Giaro, K. (2012). Matching
split distance for unrooted binary phylogenetic trees.
*IEEE/ACM Transactions on Computational Biology and
Bioinformatics*, *9*(1), 150–160. doi:10.1109/TCBB.2011.48

Lin, Y., Rajan, V., & Moret, B. M. E. (2012). A metric for
phylogenetic trees based on matching. *IEEE/ACM Transactions on
Computational Biology and Bioinformatics*, *4*(9), 1014–1022.
doi:10.1109/TCBB.2011.157

Nelson, G. (1979). Cladistic analysis and synthesis: Principles and
definitions, with a historical note on Adanson’s
*Familles des Plantes* (1763–1764).
*Systematic Biology*, *28*(1), 1–21. doi:10.1093/sysbio/28.1.1

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alternative phylogenetic trees. *Bioinformatics*,
*22*(1), 117–119. doi:10.1093/bioinformatics/bti720

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Robinson-Foulds metrics for comparing phylogenetic trees.
*Bioinformatics*, *36*(20), 5007–5013. doi:10.1093/bioinformatics/btaa614

Steel, M. A., & Penny, D. (2006). Maximum parsimony and the
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(Ed.), *Parsimony, phylogeny, and genomics* (pp. 163–178).
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