U01.21.011 Absolute Risk Reduction (ARR)

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Learning Objective:
Understand and apply the formula for calculating Absolute Risk Reduction (ARR), which quantifies the absolute difference in risk between a control group and a treatment (or exposed) group.

Formula:

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

Explanation:
In a 2×2 table:

Disease (+) Disease (–)
Treatment/Exposure (+) a b
Control/Unexposed (–) c d
  • frac{c}{c + d} = Risk in control (unexposed) group
  • frac{a}{a + b} = Risk in treatment (or exposed) group

ARR measures the actual risk reduction achieved by the intervention.

Example:

Disease (+) Disease (–)
Treatment (+) 10 90
Control (–) 20 80

ARR = frac{20}{20 + 80} - frac{10}{10 + 90} = 0.2 - 0.1 = 0.1

✅ Answer: Absolute Risk Reduction = 0.1 (10%)
→ The treatment reduces the absolute risk by 10%.

Activity:

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