I recently went back to my analysis of the Ethnologue into bilingualism. I suggested estimating the level of bilingualism by calculating the minimum number of bilinguals. However, there is already a much better estimation, and it has already been calculated. Greenberg's diversity index calculates the probability of two people who have different mother tongues in the same country meeting. This index has already been calculated for each country in the ethnologue. Below is a map of these indices. Dark colours indicate a higher diversity index (few people have the same mother tongue), lighter colours indicate a lower diversity index (total white = all people have the same mother tongue).
However, this still isn't a good predictor of bilingualism. Instead, it's a measure of diversity of mother tongue. The index still assumes all people only speak one language. I'm trying to figure out how to modify the index to take account of bilinguals - but I was always rubbish at probability.
The Ultimate Test
1 week ago