If no other group is calling at a similar level, then it's not reversion to the mean. The mean has actually been shifted downwards. > Since there was also like

If no other group is calling at a similar level, then it's not reversion to the mean. The mean has actually been shifted downwards. > Since there was also likely some underlying health condition, you would expect the between-year variance to be large. It might well be that their program has no effect whatsoever. The rate dropped significantly each year from 2015 through 2017 -- 3 years in row. It's possible this is pure chance, but it seems rather unlikely.