The formula RR = (a/(a+b)) ÷ (c/(c+d)) represents what?

• A. Relative risk
• B. Odds ratio
• C. Risk difference
• D. Prevalence ratio

Answer: (A)

This formula measures how much more or less likely the outcome is in the exposed group compared with the unexposed group. The top part, a/(a+b), is the risk (probability) of the outcome among those who are exposed. The bottom part, c/(c+d), is the risk among those who are not exposed. Dividing the exposed risk by the unexposed risk gives a ratio that tells you how many times higher (or lower) the risk is with exposure — this is relative risk.

This is typically used in cohort studies because you’re comparing incidence between two groups over time. Other measures would describe different concepts: the odds ratio uses odds rather than risks; the risk difference looks at the absolute difference between the two risks; and the prevalence ratio relates to how common the condition is at a single point in time rather than over a follow-up period.