# Number needed to treat  Main Article Discussion Related Articles  [?] Bibliography  [?] External Links  [?] Citable Version  [?] 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. The NNT has been proposed to improve quantitative literacy. The calculations are derived from the results of a randomized controlled trial of an intervention.

Two-by-two table for a screening program
Outcome
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

## Calculations

### Event rates

${\mbox{Experimental event rate}}=\left({\frac {\mbox{Cell A}}{\mbox{Cell A + Cell B}}}\right)$ ${\mbox{Control event rate}}=\left({\frac {\mbox{Cell C}}{\mbox{Cell C + Cell D}}}\right)$ ### Measures of efficacy

${\mbox{Absolute risk reduction}}=\left({\mbox{Experimental event rate}}-{\mbox{Control event rate}}\right)$ ${\mbox{Number need to treat}}=\left({\frac {100}{\mbox{Absolute risk reduction}}}\right)$ ### Deriving the NNT from the odds or risk ratios

The odds ratio may be used to derive the number needed to treat:

For odds ratios less than 1:

$NNT={\frac {1-CER*(1-OR)}{CER*(1-OR)*(1-CER)}}{\mbox{, where CER is control event rate and OR is odds ratio}}$ For odds ratios greater than 1:

$NNT={\frac {1+CER*(OR-1)}{CER*(OR-1)*(1-CER)}}{\mbox{, where CER is control event rate and OR is odds ratio}}$ The relative risk ratio may be used to derive the number needed to treat:

$NNT={\frac {1}{CER*(1-RRR)}}{\mbox{, where CER is control event rate and RRR is relative risk ratio}}$ The relative risk reduction may be used to derive the number needed to treat:

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

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