U01.21.007 Odds Ratio (OR)

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Learning Objective:
Understand and apply the formula for calculating the odds ratio (OR), which measures the strength of association between an exposure and an outcome in case-control studies.

Formula:

OR = frac{a/c}{b/d} = frac{ad}{bc}

Explanation:
In a 2×2 table:

Disease (+) Disease (–)
Exposure (+) a b
Exposure (–) c d
  • a = Exposed cases
  • b = Exposed controls
  • c = Unexposed cases
  • d = Unexposed controls

The odds ratio compares the odds of exposure among cases to the odds of exposure among controls.

  • OR = 1 → no association
  • OR > 1 → exposure associated with higher disease odds
  • OR < 1 → exposure may be protective

Example:

Disease (+) Disease (–)
Exposure (+) 40 20
Exposure (–) 10 30

OR = frac{40 times 30}{20 times 10} = frac{1200}{200} = 6

✅ Answer: Odds Ratio = 6 → The exposed group has 6 times higher odds of disease.

Activity:

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