3. Suppose you are in a planning stage of research to estimate the incidence of children allergy and explore the potential risk factors, a questionnaire survey will be administered to the parents of children aged 0–3 years, please answer the following question.

(a) Please design a sampling plan to get a representative sample from the city you are living to answer the research questions.

(b) If we assume that the “estimated” incidence of allergy is 21.3%, how can we estimate the sample size to be investigated, and what other information is needed?

(c) To investigate the potential risk factors of allergy among children, what else should be considered in sample size estimation?