Computing relative risk
Relative risk (RR) is the ratio of the probability of an event in the exposed group to the probability in the unexposed group. An RR of 1 means no difference in risk. An RR greater than 1 means the exposure increases risk; less than 1 means it decreases risk.
The 95% confidence interval is calculated using the log method. If the interval does not include 1, the result is statistically significant at the 5% level. The number needed to treat (NNT) or harm (NNH) is the reciprocal of the absolute risk difference.
RR = [a/(a+b)] / [c/(c+d)]
Relative risk from a 2x2 table where a=exposed events, b=exposed non-events, c=unexposed events, d=unexposed non-events.