Case-control studies examine the association between outcomes and potential risk factors by looking backward.
Researchers start with two groups of people: those with and those without a condition/disease. Then they collect information about how often each group was exposed to the potential cause of the condition/disease, looking to see if there’s a connection between the amount of exposure and the presence of the condition/disease.
When to include: We don’t often use case-control studies in systematic reviews, but they can be useful if your question is about how people with a given (usually rare) condition or exposure relate to those without that condition or exposure. Some diagnostic test studies also use the case-control design.