Case Control Study

TL;DR

• Observational design that compares people with a condition to people without it to identify possible risk factors.
• Can be conducted relatively quickly and inexpensively and is useful for studying rare diseases or conditions.
• Because it is retrospective and involves investigator-selected cases and controls, it is vulnerable to selection bias and recall bias.

Definition

A case-control study is a type of observational study commonly used in epidemiology to identify potential risk factors for a specific disease or condition. In a case-control study, the investigator selects a group of individuals with the disease or condition (cases) and a group of individuals without the disease or condition (controls), and then compares the two groups for potential risk factors.

Explanation

In this design the investigator first identifies cases (those with the disease or condition) and controls (those without it), then assesses and compares past exposures or characteristics between the two groups to identify associations with the disease. Case-control studies are retrospective in nature. They can be conducted relatively quickly and inexpensively compared to other study types and are particularly useful when studying rare diseases or conditions, since investigators can select cases from a large population and then compare them to controls. Limitations arise because cases and controls are selected based on disease status, which can introduce selection bias (for example, if they are chosen from different populations and thus are not representative). Being retrospective also creates potential for recall bias, where individuals with the disease may be more likely to remember or report past exposures than those without the disease.

Examples

Cigarette smoking and lung cancer

An investigator selects a group of individuals with lung cancer (cases) and a group without lung cancer (controls) and compares the two groups for cigarette smoking habits to examine whether individuals with lung cancer were more likely to be smokers than individuals without lung cancer.

Oral contraceptive use and breast cancer

An investigator selects a group of women with breast cancer (cases) and a group of women without breast cancer (controls) and compares the two groups for oral contraceptive use to examine whether women with breast cancer were more likely to have used oral contraceptives than women without breast cancer.

Use cases

• Identifying potential risk factors for specific diseases or conditions.
• Studying rare diseases or conditions where selecting cases from a large population is practical.

Notes or pitfalls

• Selection bias: choosing cases and controls from different populations can make results unrepresentative of the target population.
• Recall bias: because the study is retrospective, individuals with the disease may be more likely to remember or report potential risk factors than those without the disease.

Related terms

• Observational study
• Epidemiology
• Cases (study subjects with the disease)
• Controls (study subjects without the disease)
• Retrospective study
• Selection bias
• Recall bias