# Relative risk ratio

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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.^{[1]} This measure should be contrasted with the absolute risk reduction.

## Calculations

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 |

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:^{[2]}^{[3]}

## References

- ↑ 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. - ↑ 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]} - ↑ 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]}