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

[edit intro]

The number needed to treat (NNT) is a way of summarizing the benefit of an intervention to improve health care.[1][2] The NNT has been proposed to improve quantitative literacy.[1] The calculations are derived from the results of a randomized controlled trial of an intervention.

Two-by-two table for a screening program  | Outcome |

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Present | Absent |
| Experimental (intervention) group | Cell A | Cell B | Total in experimental group
Control group | Cell C | Cell D | Total in control group
| Total with outcome | Total without outcome |

## Contents

* 1 Calculations
* 1.1 Event rates
* 1.2 Measures of efficacy
* 1.3 Deriving the NNT from the odds or risk ratios
* 2 Example
* 3 Variations
* 4 References
* 5 See also
* 6 External links

## Calculations[edit]

### Event rates[edit]

Experimental event rate = ( Cell A Cell A + Cell B ) {\displaystyle {\mbox{Experimental event rate}}=\left({\frac {\mbox{Cell A}}{\mbox{Cell A + Cell B}}}\right)}

Control event rate = ( Cell C Cell C + Cell D ) {\displaystyle {\mbox{Control event rate}}=\left({\frac {\mbox{Cell C}}{\mbox{Cell C + Cell D}}}\right)}

### Measures of efficacy[edit]

Absolute risk reduction = ( Experimental event rate − Control event rate ) {\displaystyle {\mbox{Absolute risk reduction}}=\left({\mbox{Experimental event rate}}-{\mbox{Control event rate}}\right)}

Number need to treat = ( 100 Absolute risk reduction ) {\displaystyle {\mbox{Number need to treat}}=\left({\frac {100}{\mbox{Absolute risk reduction}}}\right)}

### Deriving the NNT from the odds or risk ratios[edit]

The odds ratio may be used to derive the number needed to treat:[3][4]

For odds ratios less than 1:[4]

N N T = 1 − C E R ∗ ( 1 − O R ) C E R ∗ ( 1 − O R ) ∗ ( 1 − C E R ) , where CER is control event rate and OR is odds ratio {\displaystyle NNT={\frac {1-CER*(1-OR)}{CER*(1-OR)*(1-CER)}}{\mbox{, where CER is control event rate and OR is odds ratio}}}

For odds ratios greater than 1:[4]

N N T = 1 + C E R ∗ ( O R − 1 ) C E R ∗ ( O R − 1 ) ∗ ( 1 − C E R ) , where CER is control event rate and OR is odds ratio {\displaystyle NNT={\frac {1+CER*(OR-1)}{CER*(OR-1)*(1-CER)}}{\mbox{, where CER is control event rate and OR is odds ratio}}}

The relative risk ratio may be used to derive the number needed to treat:[3][5]

N N T = 1 C E R ∗ ( 1 − R R R ) , where CER is control event rate and RRR is relative risk ratio {\displaystyle NNT={\frac {1}{CER*(1-RRR)}}{\mbox{, where CER is control event rate and RRR is relative risk ratio}}}

The relative risk reduction may be used to derive the number needed to treat:[3][5]

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}}}

## Example[edit]

## Variations[edit]

Years-needed-to-treat to add 1 year of life is proposed to estimate treatment effects.[6]

## References[edit]

1. ↑ 1.0 1.1 Laupacis A, Sackett DL, Roberts RS (1988). "An assessment of clinically useful measures of the consequences of treatment". N. Engl. J. Med. 318 (26): 1728–33. PMID 3374545. [e]
2. ↑ Wen L, Badgett R, Cornell J (2005). "Number needed to treat: a descriptor for weighing therapeutic options". Am J Health Syst Pharm 62 (19): 2031–6. DOI:10.2146/ajhp040558. PMID 16174840. Research Blogging.
3. ↑ 3.0 3.1 3.2 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]
4. ↑ 4.0 4.1 4.2 McQuay HJ, Moore RA (May 1997). "Using numerical results from systematic reviews in clinical practice". Ann. Intern. Med. 126 (9): 712–20. PMID 9139558. [e]
5. ↑ 5.0 5.1 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]
6. ↑ Levy WC, Mozaffarian D, Linker DT, et al. (March 2009). "Years-needed-to-treat to add 1 year of life: a new metric to estimate treatment effects in randomized trials". Eur. J. Heart Fail. 11 (3): 256–63. DOI:10.1093/eurjhf/hfn048. PMID 19164422. Research Blogging.

## See also[edit]

* Randomized controlled trial
* Absolute risk reduction
* Relative risk reduction

## External links[edit]

Tables of NNTs for various treatments:

* Bandolier
* University of Toronto Centre for Evidence-Based Medicine