What Is a Case-Control Study? | Definition & Examples

A case-control study is an experimental design that compares a group of participants possessing a condition of interest to a very similar group lacking that condition. Here, the participants possessing the attribute of study, such as a disease, are called the ‘case’, and those without it are the ‘control’.

It’s important to remember that the case group is chosen because they already possess the attribute of interest. The point of the control group is to facilitate investigation, e.g., studying whether the case group systematically exhibits that attribute more than the control group does.

When to use a case-control study

Case-control studies are a type of observational study often used in fields like medical research, environmental health, or epidemiology. While most observational studies are qualitative in nature, case-control studies can also be quantitative, and they often are in healthcare settings. Case-control studies can be used for both exploratory and explanatory research, and they are a good choice for studying research topics like disease exposure and health outcomes.

A case-control study may be a good fit for your research if it meets the following criteria.

1. Data on exposure (e.g., to a chemical or a pesticide) are difficult to obtain or expensive.
2. The disease associated with the exposure you’re studying has a long incubation period or is rare or under-studied (e.g., AIDS in the early 1980s).
3. The population you are studying is difficult to contact for follow-up questions (e.g., asylum seekers).

Examples of case-control studies

Case-control studies are common in fields like epidemiology, healthcare, and psychology.

Advantages and disadvantages of case-control studies

Case-control studies are a solid research method choice, but they come with distinct advantages and disadvantages.

Advantages of case-control studies

• Case-control studies are a great choice if you have any ethical considerations about your participants that could preclude you from using a traditional experimental design.
• Case-control studies are time efficient and fairly inexpensive to conduct because they require fewer subjects than other research methods.
• If there were multiple exposures leading to a single outcome, case-control studies can incorporate that. As such, they truly shine when used to study rare outcomes or outbreaks of a particular disease.

Disadvantages of case-control studies

• Case-control studies, similarly to observational studies, run a high risk of research biases. They are particularly susceptible to observer bias, recall bias, and interviewer bias.
• In the case of very rare exposures of the outcome studied, attempting to conduct a case-control study can be very time consuming and inefficient.
• Case-control studies in general have low internal validity and are not always credible.

Case-control studies by design focus on one singular outcome. This makes them very rigid and not generalisable, as no extrapolation can be made about other outcomes like risk recurrence or future exposure threat. This leads to less satisfying results than other methodological choices.

Frequently asked questions

What’s the difference between a case-control study and a cohort study?

A case-control study differs from a cohort study because cohort studies are more longitudinal in nature and do not necessarily require a control group.

While one may be added if the investigator so chooses, members of the cohort are primarily selected because of a shared characteristic among them. In particular, retrospective cohort studies are designed to follow a group of people with a common exposure or risk factor over time and observe their outcomes.

Case-control studies, in contrast, require both a case group and a control group, as suggested by their name, and usually are used to identify risk factors for a disease by comparing cases and controls.

What’s the difference between a case-control study and a cross-sectional study?

A case-control study differs from a cross-sectional study because case-control studies are naturally retrospective in nature, looking backward in time to identify exposures that may have occurred before the development of the disease.

On the other hand, cross-sectional studies collect data on a population at a single point in time. The goal here is to describe the characteristics of the population, such as their age, gender identity, or health status, and understand the distribution and relationships of these characteristics.

How are cases and controls selected in a case-control study?

Cases and controls are selected for a case-control study based on their inherent characteristics. Participants already possessing the condition of interest form the “case,” while those without form the “control.”

Keep in mind that by definition the case group is chosen because they already possess the attribute of interest. The point of the control group is to facilitate investigation, e.g., studying whether the case group systematically exhibits that attribute more than the control group does.

How can the strength of association between an exposure and a disease be measured in a case-control study?

The strength of the association between an exposure and a disease in a case-control study can be measured using a few different statistical measures, such as odds ratios (ORs) and relative risk (RR).

Can case-control studies establish causality?

No, case-control studies cannot establish causality as a standalone measure.

As observational studies, they can suggest associations between an exposure and a disease, but they cannot prove without a doubt that the exposure causes the disease. In particular, issues arising from timing, research biases like recall bias, and the selection of variables lead to low internal validity and the inability to determine causality.