(1) are not very good. Xiaoying investigated in the park, and the old people who are diligent in exercise are of course less sick; Xiaoling is investigating sick old people in the hospital, of course; Xiaohua method is good, but the base is too small and easy to be one-sided.
(2) Use Xiaohua's method, but need to investigate more people, such as 1000 people.
(3) Old people who are diligent in exercise are healthier and less sick.