[kqr]

> Asking unhappy users for feedback is a great idea. The 0 to 10 scale and - especially - the bizarre calculation NPS does with it - is statistical malpractice

I disagree. On the contrary, it is good survey practise to ask yes/no questions with a scale, to get information on intensity of opinion. (Often this is a scale 1--7 ranging from "strongly disagree" through "neutral" to "strongly agree", but there are other alternatives of which NPS is one.)

Then you might think only the bottom half of the range would formally be considered "detractors", and that would (as far as the evidence goes) be accurate. However, we're not looking for how many people are detractors today -- we want to know how many people are at risk of becoming detractors in the near future. That's what makes NPS a somewhat leading metric. And that's also why you see the sleight of hand that counts answers up to 6 as detractors.

I don't know what "mathematically capable" means in your context, nor what "NPS supports claim" in your experience, but the number absolutely makes sense.