As a commonly used clinical study design protocol, a cohort study is a cause-to-effect study to investigate the impact of certain exposure factors (causative factors or interventions) on disease outcome. Cohort study is to form different subgroups according to whether subjects from a specific population are exposed to certain risk factors or their exposure degree now or in the past. The researcher does not control or intervene on the exposure factors of the observed population, and after a period of follow-up, observe the differences in outcomes of each subgroup, so as to analyze the causal association between exposure factors and outcomes. It belongs to the category of observational research. It is commonly used in etiology research, therapeutic research, prognosis research, etc. Common cohort study design protocol types are as follows.