In clinical epidemiology and evidence-based medicine, the relative risk reduction 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.^[1] This measure should be contrasted with the absolute risk reduction.

Calculations[edit]

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

${\displaystyle {\mbox{Experimental event rate EER)}}=\left({\frac {\mbox{ Cell A}}{{\mbox{Total in the }}experimental{\mbox{ group}}}}\right)}$

${\displaystyle {\mbox{Control event rate (CER)}}=\left({\frac {\mbox{Cell C}}{{\mbox{Total in the }}control{\mbox{ group}}}}\right)}$

${\displaystyle {\mbox{Relative risk reduction}}=\left({\frac {\mbox{CER - EER}}{\mbox{CER}}}\right)}$

The relative risk reduction may be used to derive the number needed to treat:^[2][3]

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

Note that the relative risk reduction is that same as 1 - the relative risk ratio.

References[edit]

See also[edit]

• Randomized controlled trial
• Absolute risk reduction
• Number needed to treat
• Relative risk