Case Control Odds Ratio

Lead Author(s): Jeff Martin, MD

Case Control Odds Ratio

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, 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.


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

Capturing Events with a Case-Control Study Design

The goal of the case-control design: 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.

In case control design, Case Control Design: OR estimates Risk Ratio and Case Control Design: OR estimates Rate Ratio.

Rare Disease Assumption

The rare disease assumption says that if the incidence of the disease under study is very low Although there is no absolute number, less than 10% is often given as a definition of what is meant by rare.