relative risk reduction

Risk reduction

The relative risk reduction (RRR) quantifies the proportion of risk reduction attributable to a specific intervention or exposure as compared to a control. RRR considers the risk for disease in the exposed/intervention group and the unexposed/control group as follows:

RRR = (risk in unexposed − risk in exposed) / (risk in unexposed)

In this case, the exposed/intervention group is the DASH diet in the highest fifth, and the unexposed/control group is the DASH diet in the third fifth; the risks are reported in person-years (incidence densities). The risk of gout among subjects adopting a DASH diet in the third fifth (unexposed/control group) is 354 / 196,510 = 0.0018, or 1.8 per 1,000 person-years; the risk of gout among subjects adopting a DASH diet in the highest fifth (exposed group) is 252 / 193,455 = 0.0013, or 1.3 per 1,000 person-years. Therefore, the RRR is calculated as:

RRR = (1.8 per 1,000 person-years − 1.3 per 1,000 person-years) / 1.8 per 1,000 person-years

RRR = 0.5 / 1.8 = 0.28 (28%)

Alternately, RRR is frequently calculated by subtracting the relative risk (RR) from 1: RRR = 1 − RR. RR is the risk of developing a disease (eg, gout) in the exposed group (eg, DASH diet in highest fifth) divided by the risk of developing the same disease in the unexposed group (eg, DASH diet in third fifth). In this case, RR = 0.0013 / 0.0018 = 0.72 (72%) (Choice C). Therefore, RRR = 1 − 0.72 = 0.28.

This means that, compared to patients who adopt a diet DASH diet in the third fifth, patients who adopt a DASH diet in the highest fifth reduce their risk of gout by 28%.