How to estimate the sample size when investigating the health status of the elderly?

There are corresponding statistical formulas to determine the sample size, and different sampling methods correspond to different formulas. According to the calculation formula of sample size, we know that the size of sample size does not depend on the number of the whole population, but on the change degree of (1) research objects; (2) Required or allowable error (i.e. accuracy requirement); (3) Infer the required confidence. That is to say, when the phenomenon studied is more complex and the difference is greater, the requirement for sample size is greater; When the required precision is high and the requirement for inferability is high, the sample size is large. Therefore, if different cities infer separately, it is wrong in principle that big cities smoke more and small cities smoke less. Too much sampling in big cities is a waste, and too little sampling in small cities has no inferential value.