relative risk reduction relative risk reduction in epidemiology, the relative risk reduction (rrr) or efficacy is the relative decrease in the risk of an adverse event in the exposed group compared to an unexposed group. it is computed as ( i u − i e ) / i u -i_)/i_ , where i e is the incidence in the exposed group, and i u is the incidence in the unexposed group. if the risk of an adverse event is increased by the exposure rather than decreased, the term relative risk increase (rri) is used, and it is computed as ( i e − i u ) / i u -i_)/i_ . if the direction of risk change is not assumed, the term relative effect is used, and it is computed in the same way as relative risk increase. the group exposed to treatment (left) has the risk of an adverse outcome (black) reduced by 50% (rrr = 0.5) compared to the unexposed group (right). numerical examples risk reduction example of risk reduction quantity experimental group (e) control group (c) total events (e) ee = 15 ce = 100 115 non-events (n) en = 135 cn = 150 285 total subjects (s) es = ee + en = 150 cs = ce + cn = 250 400 event rate (er) eer = ee / es = 0.1, or 10% cer = ce / cs = 0.4, or 40% — variable abbr. formula value absolute risk reduction arr cer − eer 0.3, or 30% number needed to treat nnt 1 / (cer − eer) 3.33 relative risk (risk ratio) rr eer / cer 0.25 relative risk reduction rrr (cer − eer) / cer, or 1 − rr 0.75, or 75% preventable fraction among the unexposed pfu (cer − eer) / cer 0.75 odds ratio or (ee / en) / (ce / cn) 0.167 risk increase example of risk increase quantity experimental group (e) control group (c) total events (e) ee = 75 ce = 100 175 non-events (n) en = 75 cn = 150 225 total subjects (s) es = ee + en = 150 cs = ce + cn = 250 400 event rate (er) eer = ee / es = 0.5, or 50% cer = ce / cs = 0.4, or 40% — variable abbr. formula value absolute risk increase ari eer − cer 0.1, or 10% number needed to harm nnh 1 / (eer − cer) 10 relative risk (risk ratio) rr eer / cer 1.25 relative risk increase rri (eer − cer) / cer, or rr − 1 0.25, or 25% attributable fraction among the exposed afe (eer − cer) / eer 0.2 odds ratio or (ee / en) / (ce / cn) 1.5