Case-control study


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Case-control study


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Case-control study

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Case-control study


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Case-control study

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A researcher is attempting to understand the relationship between seatbelt use and risk for traumatic brain injury (TBI), following a motor vehicle collision (MVC) in a small set of patients presenting to a large urban emergency department. The researcher identifies 200 patients who sustained traumatic brain injury following a MVC and 200 patients with similar characteristics who did not have TBI following a MVC. The researcher then calculates the odds ratio of having TBI in patients who were wearing seatbelts versus those who did not. Which of the following best describes this study design?  

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Case-control studies p. 260

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Content Reviewers

Yifan Xiao, MD


Evan Debevec-McKenney

Tanner Marshall, MS

Case-control studies are a type of study design that compares the history of two groups of people - those that have a certain outcome, called cases, and those that don’t have a certain outcome, called controls; to see if they’ve been exposed to different things.

For example, a case-control study might find that the odds of using tanning beds is higher for people with skin cancer, the cases, compared to people without skin cancer, the controls.

Because case-control studies look at the past exposures of people with and without the outcome, this type of study is called retrospective, retro meaning past.

The opposite would be to start with people who either have the exposure or don’t have the exposure, and then follow them over time to see if they develop an outcome in the future.

That would be a prospective study.

But, following people over time can take a lot of time and cost money.

By comparison, case-control studies are often quicker and cheaper, since they use data that’s already collected or is relatively easy to collect, like medical records, employment records, or individual interviews and surveys.

For example, researchers might interview a group individuals with skin cancer and a group of individuals without skin cancer to see how many times they used tanning beds in the past five years, then compare the results.

This is particularly true when it comes to rare diseases, but the definition of “rare” varies around the world.

For example, in the United States, 1 in 1,500 people is considered rare, but in Japan, 1 in 2,500 people is considered rare.

Either way, it’s easier to recruit individuals with rare diseases into a study, rather than start with a group of healthy individuals and wait to see who develops a rare disease in the future.

For example, in the United States, you’d have to follow a group of 30,000 healthy individuals over time, to identify 20 people who develop a rare disease.

Case-control studies are also useful in situations that may be unethical to test using other approaches.

For example, a case-control study could be used to find out if people with skin cancer happened to use tanning beds more than people without skin cancer.

But doing a randomized control trial, where individuals are assigned to tanning or no tanning might be unethical, especially if we think that tanning causes skin cancer.


A case-control study is an observational method used to compare a group of individuals with a particular condition (the cases) to another, a similar group of people without that condition (the controls). The investigation begins after researchers have identified a group of people with the condition they wish to study. A second, comparable group who does not have the condition is then identified from medical records or other sources. Investigators then look back through both groups' records to identify possible causes or risk factors for the condition. Case-control studies can be conducted relatively quickly and cheaply since researchers don't have to follow participants over time as they would in a cohort study. Another advantage is that case-control studies can be used to study rare conditions that would be challenging in a cohort study.


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