# nnt: Calculate the number needed to treat (NNT) In meta: General Package for Meta-Analysis

## Description

Calculate the number needed to treat (NNT) from estimated risk difference, risk ratio, or odds ratio, and a baseline risk.

## Usage

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18``` ```nnt(x, ...) ## S3 method for class 'meta' nnt(x, p.c, comb.fixed = x\$comb.fixed, comb.random = x\$comb.random, ...) ## Default S3 method: nnt(x, p.c, sm, lower, upper, ...) ## S3 method for class 'nnt.meta' print( x, comb.fixed = x\$comb.fixed, comb.random = x\$comb.random, digits = gs("digits"), digits.prop = gs("digits.prop"), big.mark = gs("big.mark"), ... ) ```

## Arguments

 `x` An object of class `meta`, or estimated treatment effect, i.e., risk difference(s), risk ratio(s), or odds ratio(s). `...` Additional arguments (ignored at the moment). `p.c` Baseline risk (control group event probability). `comb.fixed` A logical indicating whether NNTs should be calculated based on fixed effect estimate. `comb.random` A logical indicating whether NNTs should be calculated based on random effects estimate. `sm` Summary measure. `lower` Lower confidence interval limit. `upper` Upper confidence interval limit. `digits` Minimal number of significant digits, see `print.default`. `digits.prop` Minimal number of significant digits for proportions, see `print.default`. `big.mark` A character used as thousands separator.

## Details

The number needed to treat (NNT) can be easily computed from an estimated risk difference (RD), risk ratio (RR), or odds ratio (OR) and a given baseline risk (Higgins & Green, 2011, section 12.5).

Accordlingly, this function can be used to calculate NNTs for meta-analyses generated with `metabin` or `metagen` if argument `sm` was equal to `"RD"`, `"RR"`, or `"OR"`. It is also possible to directly provide estimated treatment effects without conducting a meta-analysis (see Examples).

The baseline risk can be specified using argument `p.c`. If this argument is missing, the minimum, mean, and maximum of the control event probabilities in the meta-analysis are used for `metabin`; otherwise the control event probabilities 0.1, 0.2, ..., 0.9 are used.

## Author(s)

Guido Schwarzer sc@imbi.uni-freiburg.de

## References

Higgins, J.P.T and S. Green (2011): Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [Updated March 2011]. The Cochrane Library: http://www.cochrane-handbook.org

`metabin`, `metagen`
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17``` ```# Calculate NNT for RD = -0.21 # (Cochrane Handbook, version 5.1, subsection 12.5.4.1) nnt(-0.21, sm = "RD") # Calculate NNT for RR = 0.92 and baseline risk p.c = 0.3 # (Cochrane Handbook, version 5.1, subsection 12.5.4.2) nnt(0.92, p.c = 0.3, sm = "RR") # Calculate NNT for OR = 0.73 and baseline risk p.c = 0.3 # (Cochrane Handbook, version 5.1, subsection 12.5.4.3) nnt(0.73, p.c = 0.3, sm = "OR") # Use Mantel-Haenszel odds ratio to calculate NNTs data(Olkin1995) m1 <- metabin(ev.exp, n.exp, ev.cont, n.cont, data = Olkin1995, comb.random = FALSE) nnt(m1, comb.random = TRUE) ```