When to calculate relative risk?
the Relative Risk (RR) of an event is the ratio between the probability of the event among the exposed and the probability of the event among the unexposed. In our example the probability of death is 30/100 = 0.3 in the exposed and 10/100 = 0.1 in the unexposed. The RR is therefore equal to 0.3/0.1 = 3.
The risk difference: this is the difference between the risk in the intervention group and the risk in the control group: Ri – Rc. If the risk decreases, this difference is called the absolute risk reduction (ARR), if the risk increases, it is the absolute increase in risk (AAR).
If the relative risk = 1, then there is no difference in terms of risk between the two groups. If the relative risk is less than 1, then it is less in the exposed group than in the unexposed group. If the relative risk is greater than 1 (as in our example), then the exposed group is…
In other words, the absolute risk increase was only 1 in 7,000, while the relative risk increase was, effectively, 100 percent.
Usually, we will compare the value of the relative risk estimated at 1, in order to be able to conclude that there is an increased risk (RR >1) or reduced (RR<1). The null hypothesis tested is: The Wald test consists in rejecting the null hypothesis if the value 1 does not belong to the confidence interval.