I don’t get your point. Maybe we agree. If in one city there are 1000 30-year-old daily-shopers, 1000 50-year-old weekly-shoppers and 1000 70-year-old monthly-s

I don’t get your point. Maybe we agree. If in one city there are 1000 30-year-old daily-shopers, 1000 50-year-old weekly-shoppers and 1000 70-year-old monthly-shoppers it will not be a good idea to estimate the age of the population by sampling people who enters the store. The average age is 50, not 34.