Definition and calculation of odds ratio

Definition and calculation of odds ratio - Cracking the Odds

• Definition: The odds of an outcome in an exposed group divided by the odds of that outcome in a non-exposed group.
• Primary Use: Case-control studies.

-   $OR = (a/b) / (c/d) = ad/bc$
-   📌 Mnemonic: **'cross-product'** for ad/bc.

• Interpretation:
  • $OR > 1$: Exposure ↑ odds of disease.
  • $OR < 1$: Exposure ↓ odds of disease (protective effect).
  • $OR = 1$: No association between exposure and disease.

⭐ The odds ratio is a good approximation of the relative risk (RR) when the disease prevalence is low (typically <10%).

High-Yield Points - ⚡ Biggest Takeaways

• The Odds Ratio (OR) is the primary measure of association in case-control studies, comparing the odds of exposure in cases vs. controls.
• It is calculated using the cross-product from a 2x2 table: OR = ad/bc.
• OR > 1 suggests an increased odds of disease with exposure, while OR < 1 suggests a protective effect.
• The OR approximates the relative risk (RR) when the disease prevalence is low (e.g., <10%).
• Unlike RR, the OR can overestimate the strength of an association, especially in common diseases.