Relative risk reduction

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. 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

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

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

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

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

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

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

See also

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