Case Control Odds Ratio

Lead Author(s): Jeff Martin, MD

Odds ratio (OR) of exposure in cases and controls is the measure of association in case-control studes

Since the case-control design starts with the cases and controls and fixes the ratio of the two by selecting one or more controls per case,

The investigator is looking within the disease and non-disease groups at the proportion in each who have the exposure of interest.

OR of exposure = OR of disease

The odds ratio is the OR of exposure,

To understand the OR in a case-control study we need to look at how the OR approximates the risk ratio and approximates the rate ratio.

Case-control Design Notation

To switch from the notation of a cohort to case-control design, the events (diagnoses) in a case-control study are the cases.

1 = EXPOSED and 0 = NOT EXPOSED

With this notation we can look at how the OR estimates other ratio measures.

The goal of the case-control design:

Obtain all the cases that occur during a defined time period (or a random sample of those cases)

Easily achievable if all, or nearly all the cases are diagnosed and registered:

Use a primary study base such as

Getting an unbiased estimate of this ratio is not a problem in a case-control design.

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.

Although there is no absolute number, less than 10% is often given as a definition of what is meant by rare.

Rare disease assumption if disease incidence low in unexposed and exposed (<10%).
- OR approximately equals RR
- Exposure in controls approximately equals exposure in whole cohort