The relative risk (RR), its standard error and 95% confidence interval are calculated according to Altman, 1991.
The relative risk or risk ratio is given by
with the standard error of the log relative risk being
and 95% confidence interval
Where zeros cause problems with computation of the relative risk or its standard error, 0.5 is added to all cells (a, b, c, d) (Pagano & Gauvreau, 2000; Deeks & Higgins, 2010).
The number needed to treat (NNT) is the estimated number of patients who need to be treated with the new treatment rather than the standard treatment (or no treatment) for one additional patient to benefit (Altman 1998).
A negative number for the number needed to treat has been called the number needed to harm.
MedCalc uses the terminology suggested by Altman (1998) with NNT(Benefit) and NNT(Harm) being the number of patients needed to be treated for one additional patient to benefit or to be harmed respectively.
The 95% confidence interval is calculated according to Daly (1998) and is reported as suggested by Altman (1998).