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Case Control Study Example

Rare Disease

Lead Author(s): Jeff Martin, MD

Rare Disease Assumption

The rare disease assumption says that if the incidence of the disease under study is very low
  • the OR from a case-control study even with sampling prevalent controls approximates the risk ratio.

Example of Rare Disease

Davis presents the following example of a rare disease, toxic shock syndrome.
  • This is an example of extremely rare disease: only 6 per 100,000 person-years

"Surveillance for toxic-schock syndrome (TSS) in Wisconsin detected 38 cases with onsets from September 1975 through June 1980.... 35 patients were matched for age and menstruation to 105 controls: 34 of 35 cases (versus 80 of 105 controls) used tampons during every menstrual period (OR - 10.6, p < 0.01).... In Wisconsin the minimum incidence of TSS as defined by clinical criteria is 6.2 cases per 100,000 menstruating women per year."

OR Is Good Approximation of RR

The odds ratio of 10.6 is a very good approximation of the risk ratio in this instance.


Davis, J. P., Chesney, P. J., Wand, P. J., & LaVenture, M. (1980). Toxic-shock syndrome: epidemiologic features, recurrence, risk factors, and prevention. N Engl J Med, 303(25), 1429-1435.

Topic revision: r4 - 11 Jun 2009 - 14:57:04 - MaryB?

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