A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls).
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Two out of 10 people
About Case-Control Studies
Case-control studies are in which a group of people who have a particular disease are observed to see whether their past exposures to some factors differ from those of a similar group who do not have the disease. They usually involve smaller numbers of people.
They can be either population based – all the cases and controls are randomly selected from the same defined geographical population – or hospital based – all the cases and controls are selected from people attending a particular hospital.
The population-based case-control study yields stronger evidence than one that is hospital based, which has many more sources of bias.