Case-control study

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In epidemiology, case-control studies are "studies which start with the identification of persons with a disease of interest and a control (comparison, referent) group without the disease. The relationship of an attribute to the disease is examined by comparing diseased and non-diseased persons with regard to the frequency or levels of the attribute in each group."[1][2]

Alternatively stated, a case control can be a backwards, or reverse, study and has been called a trohoc study (cohort spelled backwards) to emphasize this.[3] Thus, in the case-control study, subjects are gathered after outcomes have developed and divided into two groups: those who had the outcome (or disease) and those who did not. The investigator then goes back in time to measure whether those who developed disease had more exposure to the putative cause.

This design is in contrast to the randomized controlled trial or cohort study in which patients are divided into an exposure group and a control group and then are prospectively followed in order to measure frequency of outcomes.


The case-control study is susceptible to many biases.[4]

Selection bias in control groups is a problem.[5][6]


Variation on the case-control study include:[7]

  • Nested case control is also called a 'case-control in a cohort study'[8]
  • Cross-sectional study has no time dimension. This design may be used for studies of diagnostic accuracy.[9]
  • Proportional mortality study
  • Ecological study (the unit of observation is a group of people)


  1. Anonymous (2022), Case-control study (English). Medical Subject Headings. U.S. National Library of Medicine.
  2. Barker, D. J. P.; Coggon, D.; Rose, Geoffrey (1997). “Case-control and cross sectional studies”, Epidemiology for the uninitiated. London: BMJ Pub. Group. ISBN 0-7279-1102-3. 
  3. Feinstein AR (1973). "Clinical biostatistics. XX. The epidemiologic trohoc, the ablative risk ratio, and "retrospective" research". Clin. Pharmacol. Ther. 14 (2): 291–307. PMID 4695389. [e]
  4. (1979) "The case-control study: consensus and controversy". J Chronic Dis 32 (1-2): 1–144. PMID 36403. [e]
  5. Grimes DA, Schulz KF (2005). "Compared to what? Finding controls for case-control studies.". Lancet 365 (9468): 1429-33. DOI:10.1016/S0140-6736(05)66379-9. PMID 15836892. Research Blogging.
  6. Schulz KF, Grimes DA (2002). "Case-control studies: research in reverse.". Lancet 359 (9304): 431-4. DOI:10.1016/S0140-6736(02)07605-5. PMID 11844534. Research Blogging.
  7. Rothman, Kenneth J. (1986). Modern epidemiology. Boston: Little, Brown, 52,70. ISBN 0-316-75776-4. 
  8. Ernster VL (1994). "Nested case-control studies.". Prev Med 23 (5): 587-90. DOI:10.1006/pmed.1994.1093. PMID 7845919. Research Blogging.
  9. St John DJ, Young GP, McHutchison JG, Deacon MC, Alexeyeff MA (1992). "Comparison of the specificity and sensitivity of Hemoccult and HemoQuant in screening for colorectal neoplasia.". Ann Intern Med 117 (5): 376-82. PMID 1503328.