When someone asks me about something statistical I always panic, if only internally. It's not my strong point and that's why I'm a dentist and not a statistician.
Somehow on clinic on Friday, a student and I got to looking at the 5 year results on the Hall technique trial taking place in Scotland (link here). I was waxing on, as I do, about how great the results looked - major failures (i.e. irreversible pulpitis, loss of vitality, abscess, or unrestorable tooth) were 3% in teeth treated with the Hall technique and 16.5% for those with conventional treatment. The p value was 0.000488 - highly significant. And then it said "NNT 8". That's when the question came - what is NNT?