I have a data set which has individual customer wise metrics like 'Average Days between purchases (ADT)' . Suppose if a customer make two purchases in 15 days then ADT will be 15. So if want to compute the ADT at a product or country level, how should i do it. Currently I am taking an average ADT of all the customers belonging to that product or market. But wanted to cross check if this is the perfect way of doing it.