Linguistic diversity index

Linguistic diversity index (LDI) may refer to either Greenberg's (language) Diversity Index^[1] or the related Index of Linguistic Diversity (ILD) from Terralingua, which measures changes in the underlying LDI over time.^[2]

Greenberg's Diversity Index (LDI) is the probability that two people selected from the population at random will have different mother tongues; it therefore ranges from 0 (everyone has the same mother tongue) to 1 (no two people have the same mother tongue).^[3] The ILD measures how the LDI has changed over time; a global ILD of 0.8 indicates a 20% loss of diversity since 1970, but ratios above 1 are possible, and have appeared in regional indexes.^[4]

The computation of the diversity index is based on the population of each language as a proportion of the total population. The index cannot fully account for the vitality of languages. Also, the distinction between a language and a dialect is fluid and often political. A great number of languages are considered to be dialects of another language by some experts and separate languages by others. The index does not consider how different the languages are from each other, nor does it account for second language usage; it considers only the total number of distinct languages, and their relative frequency as mother tongues.^[5]

A few examples may be helpful in understanding these caveats.

Are Cantonese and Mandarin Chinese considered the same language for this survey? A footnote in the UNESCO report says only "Country for which a comparison between Ethnologue’s official language designation and IBE’s coded timetable information was possible, the result of which highlighted certain inconsistencies."

Haiti is listed with an index of 0.000, despite having both French and Haitian Creole as official languages.

The United States is listed with an index of 1/3, which mechanically implies that if you pick two people at random, at least 1/3 of the time one of them won't have English as their Mother tongue -- but that is very different from saying that they are not fluent in English.