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This editable Main Article is under development and subject to a disclaimer.

[edit intro]

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 |

Experimental event rate EER) = ( Cell A Total in the e x p e r i m e n t a l group ) {\displaystyle {\mbox{Experimental event rate EER)}}=\left({\frac {\mbox{ Cell A}}{{\mbox{Total in the }}experimental{\mbox{ group}}}}\right)}

Control event rate (CER) = ( Cell C Total in the c o n t r o l group ) {\displaystyle {\mbox{Control event rate (CER)}}=\left({\frac {\mbox{Cell C}}{{\mbox{Total in the }}control{\mbox{ group}}}}\right)}

Relative risk reduction = ( CER - EER CER ) {\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]

N N T = 1 C E R ∗ ( R R R ) , where CER is control event rate and RRR is relative risk reduction {\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]

1. ↑ Barratt A, Wyer PC, Hatala R, et al (2004). "Tips for learners of evidence-based medicine: 1. Relative risk reduction, absolute risk reduction and number needed to treat". CMAJ 171 (4): 353–8. DOI:10.1503/cmaj.1021197. PMID 15313996. Research Blogging.
2. ↑ Furukawa TA, Guyatt GH, Griffith LE (February 2002). "Can we individualize the 'number needed to treat'? An empirical study of summary effect measures in meta-analyses". Int J Epidemiol 31 (1): 72–6. PMID 11914297. [e]
3. ↑ Chatellier G, Zapletal E, Lemaitre D, Menard J, Degoulet P (February 1996). "The number needed to treat: a clinically useful nomogram in its proper context". BMJ 312 (7028): 426–9. PMID 8601116. PMC 2350093. [e]

## See also[edit]

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