B. It is the usual outcome measure of cohort studies – Explanation
Remember that risk and odds are different. If 20 patients die out of every 100 who have a
myocardial infarction then the risk of dying is 20 / 100 = 0.2 whereas the odds are 20 / 80 = 0.25.
Relative risk
Relative risk (RR) is the ratio of risk in the experimental group (experimental event rate, EER) to
risk in the control group (control event rate, CER). The term relative risk ratio is sometimes used
instead of relative risk.
To recap
- EER = rate at which events occur in the experimental group
- CER = rate at which events occur in the control group
For example, if we look at a trial comparing the use of paracetamol for dysmenorrhoea compared to
placebo we may get the following results
Total number of patients Experienced significant pain relief
| Total number of patients | Achieved = 50% pain relief | |
| Paracetamol | 100 | 60 |
| Placebo | 80 | 20 |
Experimental event rate, EER = 60 / 100 = 0.6
Control event rate, CER = 20 / 80 = 0.25
Therefore the relative risk ratio = EER / CER = 0.6 / 0.25 = 2.4
If the risk ratio is > 1 then the rate of an event (in this case experiencing significant pain relief) is
increased compared to controls. It is therefore appropriate to calculate the relative risk increase if
necessary (see below).
