Rare disease assumption

The rare disease assumption is a useful mathematical assumption in epidemiologic case control studies where the hypothesis tests the association between an exposure and a disease. It is assumed that, if the prevalence of the disease is low, then the odds ratio approaches the relative risk.

Case control studies are relatively inexpensive and less time consuming than cohort studies. Since case control studies don't track patients over time, they can't establish relative risk. The case control study can, however, calculate the exposure-odds ratio, which, mathematically, approaches the relative risk as prevalence falls.

References

• Greenland S, Thomas DC (September 1982). "On the need for the rare disease assumption in case-control studies". Am. J. Epidemiol. 116 (3): 547–53. PMID 7124721. http://aje.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=7124721. 

• Cummings P, Koepsell TD (September 2001). "On the need for the rare disease assumption in some case-control studies". Inj. Prev. 7 (3): 254. doi:10.1136/ip.7.3.254-a. PMC 1730752. PMID 11565997. http://ip.bmj.com/cgi/pmidlookup?view=long&pmid=11565997. 

• Greenland S, Thomas DC, Morgenstern H (December 1986). "The rare-disease assumption revisited. A critique of "estimators of relative risk for case-control studies"". Am. J. Epidemiol. 124 (6): 869–83. PMID 3776970. http://aje.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=3776970. 

• Bjerre LM, LeLorier J (February 2000). "Expressing the magnitude of adverse effects in case-control studies: "the number of patients needed to be treated for one additional patient to be harmed"". BMJ 320 (7233): 503–6. doi:10.1136/bmj.320.7233.503. PMC 1127536. PMID 10678870. http://bmj.com/cgi/pmidlookup?view=long&pmid=10678870.