Calculating relative risk
In a cohort study, 30 of every 1,000 smokers and 10 of every 1,000 nonsmokers develop a disease. What is the relative risk for smokers?
Answer C: explanation for every choice
A
This inverts the ratio, dividing the risk in nonsmokers by the risk in smokers.
B
This is the relative increase, (30 − 10)/10, which equals the relative risk minus 1.
C
Correct. Relative risk = risk in exposed / risk in unexposed = 0.030/0.010 = 3.0.
D
20 per 1,000 is the risk difference (attributable risk), not a ratio.
TakeawayRelative risk = risk(exposed) / risk(unexposed). Risk difference = risk(exposed) − risk(unexposed).
Equation usedRelative risk
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