Odds Ratio

Definition

The odds ratio (OR) compares the likelihood of an event occurring in an exposed group versus a non-exposed group. It is calculated by dividing the odds of exposure among cases by the odds of exposure among non-cases.

How it works

The odds ratio is the standard measure of association in case-control studies where the temporal relationship cannot be established directly. Unlike the relative risk, OR relies on odds. When the disease is rare, OR approximates RR well, making it useful for estimates. OR has the advantage of being symmetric: it can be calculated in the same way to compare exposure between cases and non-cases or vice-versa. This is the main measure used in retrospective nutritional epidemiology.

Role

Quantifies the association between past nutritional exposure and current disease in case-control studies

Examples

• Odds ratio of 2.5 for obesity among regular fast-food consumers
• Odds ratio of 0.6 for cardiovascular diseases among daily fruit eaters

Recommendations

Do not confuse OR and RR: OR overestimates risk when the disease is common. Use OR predominantly in case-control studies. In cross-sectional studies, prefer the prevalence ratio over OR.

Key takeaway

The odds ratio measures relative odds of disease between exposed and non-exposed groups, particularly useful in retrospective studies

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