Work out relative risk instantly with clear inputs, formula shown and shareable results.
Relative risk divides the incidence in the exposed group by that in the unexposed. The absolute risk difference matters more for decisions, and its reciprocal is the number needed to treat or harm.
Relative risk
RR = (a/n₁) / (c/n₂)
Number needed
NNT = 1 / |risk difference|
RR = 3 with a risk difference of 0.2, so one extra case arises for every 5 people exposed.
A relative risk of 3 sounds alarming, but if the baseline risk is 1 in a million the absolute effect is negligible.