I think a lot of us are curious about questions like "how different are U.S. and British English?" and "how different are Colombian and Argentinian Spanish?"
Haven't any of the academic linguists, philologists, phonologists ever some up with some kind of numerical measure of the "distance" between national varieties of a language?
There would seem to be all sorts of approaches that could be taken... for example, show a U.S. speaker a list of the 5,000 most common U.K. English words and see how many the U.S. speaker identified as "British," or how many of the were listed as "Usu. Brit." in a U.S. dictionary.
Dan, "dialectometry" (the word I found a minute after reading your question) may deserve googling it.
But have in mind, that
- it looks like 'ad hoc' definition of ones activity rather than a known term. I didn't know it (and I'm not that virgin in the field). Google provides just 10k results for it, mosly concerning German dialects.
- dialectologists are concerned with things less vague than 'national varieties' and less worldly than nations.