Study design is the framework used to collect and analyze data for clinical studies. Clinical studies can be descriptive or analytical. In descriptive studies, we describe the clinical characteristics of a target group of subjects without comparison to another group of subjects. These studies are usually hypothesis generating. Some examples include case report and case series. In contrast, when performing analytical studies, we aim to test a hypothesis and examine the relationship between exposures and outcome. Comparisons or control groups exist in analytical studies. Some examples of analytical studies include randomized controlled trials, cohort studies, and case-control studies. Analytical studies can further be divided into experimental studies and observational studies. In experimental studies, the investigators assign the exposures of the subjects, with or without randomization. Experimental studies allow the assessment of possible causal relationship between exposure and outcome. In observational studies, the investigators observe the naturally occurred exposures of the subjects instead of assigning the exposures. Case-control study is an example of observational study. The purpose of case-control study is to examine exposures associated with diseases or outcomes. Cases are individuals who had the outcome of interest, while controls are those who did not have the outcome. Cases and controls may have the exposures to different extent. Cases and controls are often be matched on some basic demographics including age and gender. In this session, we will go through each of the terminology about study design in more detail. We will discuss the advantages and disadvantages of case-control study, and the methods to study association. We will also briefly introduce the concept of nested case-control study.