Prevalence rate is a measure of the existence or prevalence of diseases; Incidence is a measure of the occurrence of diseases.
Objective: ① Chronic diseases are commonly used in cross-sectional studies; ② Reflect the burden of disease; (3) To provide basis for rational planning of health facilities, manpower, material resources and health resources; ④ Study the epidemic factors of diseases; ⑤ Monitoring the control effect of chronic diseases.