The odds ratio (OR), its standard error and 95% confidence interval are calculated according to Altman, 1991.
The odds ratio is given by
with the standard error of the log odds ratio being
and 95% confidence interval
Where zeros cause problems with computation of the odds ratio or its standard error, 0.5 is added to all cells (a, b, c, d) (Pagano & Gauvreau, 2000; Deeks & Higgins, 2010).