# Relative risk ratio  Main Article Discussion Related Articles  [?] Bibliography  [?] External Links  [?] Citable Version  [?] This editable Main Article is under development and subject to a disclaimer. [edit intro]

In clinical epidemiology and evidence-based medicine, the relative risk ratio or more simply the relative risk, is a measure of the likelihood of a clinical outcome in group of patients exposed to a factor compared to a control group of patients. This measure should be contrasted with the absolute risk reduction.

## Calculations

Two-by-two table for a randomized-controlled trial or cohort study
Outcome
Present Absent
Experimental group Cell A Cell B Total in the experimental group
Control group Cell C Cell D Total in the control group
Total with the outcome Total without the outcome
${\mbox{Experimental event rate EER)}}=\left({\frac {\mbox{ Cell A}}{{\mbox{Total in the }}experimental{\mbox{ group}}}}\right)$ ${\mbox{Control event rate (CER)}}=\left({\frac {\mbox{Cell C}}{{\mbox{Total in the }}control{\mbox{ group}}}}\right)$ ${\mbox{Relative risk}}=\left({\frac {\mbox{EER}}{\mbox{CER}}}\right)$ Note that the relative risk ratio is that same as 1 - the relative risk reduction.

The relative risk ratio may be used to derive the number needed to treat:

$NNT={\frac {1}{CER*(1-RRR)}}{\mbox{, where CER is control event rate and RRR is relative risk ratio}}$ 