U01.21.008 Relative Risk (RR)

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Learning Objective:
Understand and apply the formula for calculating Relative Risk (RR), which measures the likelihood of developing a disease in the exposed group compared to the unexposed group in cohort studies.

Formula:

RR = frac{a / (a + b)}{c / (c + d)}

Explanation:
In a 2×2 table:

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

The relative risk compares the probability (risk) of disease in the exposed versus unexposed group.

  • RR = 1 → No association
  • RR > 1 → Exposure increases risk
  • RR < 1 → Exposure decreases risk (protective)

Example:

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

RR = frac{40 / (40 + 60)}{20 / (20 + 80)} = frac{0.4}{0.2} = 2

✅ Answer: Relative Risk = 2 → The exposed group is twice as likely to develop the disease.

Activity:

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