This study begins with cases (diseased subjects) and the controls (non-diseased subjects) and then investigates their exposure status.
Case-control studies are performed when the population at risk is not known or it is impractical to follow up the whole population at risk. Remember that controls should have the same chance of being exposed to the risk factors as the cases.
These studies are quick and often the first line of enquiry into any rare disease or newly emerging disease of uncertain aetiology. For example Human BSE, Legionnaire’s disease, AIDS, Severe acute respiratory disease (SARS). All of these conditions were initially investigated using case-control studies.
Analysis of case-control studies
In a case-control study there is a group of subjects with the disease (cases) and a sample of subjects without the disease (controls). The exposure status for each subject is then determined and compared in the two groups.
Please note that it is not possible to estimate the incidence of disease from a case-control study unless the study is population based and all cases in a defined population are obtained.
The odds ratio (OR) is a measure of association for a case-control study. It tells us how much higher the odds of exposure is amongst cases of a disease compared with controls. The odds ratio compares the odds of exposure to the factor of interest among cases to the odds of exposure to the factor among controls.
Books and Journals
|Farmer R. and Lawrenson R.(2002). Epidemiology in public health. Blackwell Publishing, 5th edition.|
|Silman A. J. (2002). Epidemiological Studies: A practical guide (2nd edn). Cambridge: Cambridge University Press.|
|Willocks L., Crampin A., Milne L., Seng C., Susman M., Gair R., Moulsdale M., Shafi S., Wall R., Wiggins R., Lightfoot N. (1998) A large outbreak of cryptosporidiosis associated with a public water supply from a deep chalk borehole. . Outbreak Investigation Team Commun Dis Public Health. Dec;1(4):239-43.|