U01.21.009 Attributable Risk (AR)

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Learning Objective:
Understand and apply the formula for calculating Attributable Risk (AR), which measures the difference in disease risk between exposed and unexposed groups in cohort studies.

Formula:

AR = frac{a}{a + b} - frac{c}{c + d}

Explanation:
In a 2×2 table:

Disease (+) Disease (–)
Exposure (+) a b
Exposure (–) c d
  • frac{a}{a + b} = Risk of disease in the exposed group
  • frac{c}{c + d} = Risk of disease in the unexposed group

The Attributable Risk quantifies the excess risk of disease that can be attributed to the exposure.

  • AR = 0 → No difference in risk
  • Positive AR → Exposure increases risk
  • Negative AR → Exposure may be protective

Example:

Disease (+) Disease (–)
Exposure (+) 40 60
Exposure (–) 20 80

AR = frac{40}{40 + 60} - frac{20}{20 + 80} = 0.4 - 0.2 = 0.2

✅ Answer: Attributable Risk = 0.2 (or 20%)
→ 20% of disease cases in the exposed group are attributable to the exposure.

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