What metrics can be used to gauge language similarity?

Question

What metrics exist to describe relative language similarity?

I've sometimes heard Spanish and Portuguese said to be something like "70% mutually intelligible." Although mutual intelligibility in this case isn't a fixed number, as native Portuguese speakers generally have an easier time understanding a Spanish speaker than vice-versa.

I've also heard that English borrows anywhere from 29% to 70% of it's vocabulary from French. But English and French are nowhere near mutually intelligible (not even 29%).

So are there any metrics that are or can be used to gauge similarity, particularly in the area of SLA?

Answer 1

One metric for how similar languages are to each other is based on the concept of Linguistic Distance. Unlike some approximations that are more subjective (ie. This languages sounds a lot like that language), linguistic distance is quantifiable. This is important because it gives a concrete comparison of two languages.

Using a statistical approach (called lexicostatistics) by comparing each language's mass of words, distances can be calculated between them. In technical terms, what is calculated is the Levenshtein distance. Based on this, one study compared both Afrikaans and West Frisian with Dutch to see which was closer to Dutch. It determined that the Dutch and Afrikaans (mutual distance of 20.9%) were considerably closer than Dutch and West Frisian (mutual distance of 34.2%).

Another metric for comparison is through historical linguistic analysis. This is based on how close two languages are in derivation from some common ancestor and how long ago they diverged. This is less well defined and there are not many quantifiable ways to compare two languages.

Answer 2

All credit to Mitch for this answer, but a very similar question was posted to Linguistics.SE:

There are qualitative and quantitative measures for 'distance'.

Qualitatively, many languages are easier to compare simply by know something about their family tree, which is implicitly recognizable (if one is lucky enough to know so many languages so well), by comparing vocabulary lists, syntax rules/systems, and phonology. The more common elements the closer they are. Mandarin and Cantonese are not mutually intelligible, but both have rich tonal systems, similar isolating grammars, and fairly simple phonological rules to map cognate words. Likewise French and Italian have a set of comparisons (similar conjugation/gender rules) and fairly regular rules mapping cognates. And the two sets of differences, those between Mandarin/Cantonese, and those between French/Spanish are of the same scale. The difference between French and German is so much larger than between Mandarin/Cantonese that one would be hard pressed to say they are of similar distance.

Quantitatively, one can define distance mathematically, by collecting a set of quantitative features in each language, and then defining a distance function that calculates an exact positive real number out of a combination of all the differences of these features. Then one can do a simple numerical comparison, the distance between X and Y and the distance between Z and W. One can go further and use a clustering algorithm to create a formal family tree of all languages of concern.

Presumably, the utility of a distance function would be to predict the difficulty in language learning or translation (machine or human); the further the distance, the more changes would need to be navigated. At some point, a distance will be meaningless; the difference between French and Swahili is just too much to settle qualitatively or quantitatively to then bother comparing with some other distance.

Alternatively, this scientific report suggests that:

The distance between two languages may also depend on whether it is in the written or spoken form. For example, the written form of Chinese does not vary among the regions of China, but the spoken languages differ sharply. Alternatively, two languages that may be close in the spoken form may differ more sharply in the written form (for example, if they use different alphabets, as in the case of German and Yiddish). Perhaps the way to address the distance between languages is not through language trees which trace the evolution of languages, but by asking a simpler question: How difficult is it for individuals who know language A to learn languages B1 through Bi, where there are i other languages.

If it is more difficult to learn language B1, than it is to learn language B2, it can be said that language B1 is more “distant” from A than language B2. Language B3 may be as difficult to learn as is language B1 for a language A speaker, but that does not mean that language B3 is close to language B1. Indeed, it may be further from B1 than it is from A. Alternatively, if the issue is the adjustment of immigrants speaking languages B1 through Bi in the linguistic destination A, one would want to know how difficult it is for speakers of B1 through Bi to learn language A.

Here are two images of the table that shows the linguistic proximity between languages according to the study.