Furukawa and Leucht (2011) give a formula for calculating the number needed to treat (NNT), i.e., (p. 1)
“the number of patients one would need to treat with the intervention in question in order to have one more success (or one less failure) than if treated in the control intervention”
based on the control group event rate (CER; for instance proportion of cases showing recovery) and Cohen’s d – an effect size in standard deviation units.
R code below:
NNT = function(d, CER) {
1 / ( pnorm( d - qnorm(1-CER) ) - CER )
}
Reference
Furukawa, T. A., & Leucht, S. (2011). How to obtain NNT from Cohen’s d: comparison of two methods. PloS one, 6(4), e19070.