B. 20% – Explanation
| Relative risk reduction = (EER – CER) / CER |
Experimental event rate, EER = 40 / 200 = 0.2
Control event rate, CER = 100 / 400 = 0.25
Relative risk reduction = (EER – CER) / CER = (0.2 – 0.25) / 0.25 = -0.2 or a 20% reduction
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 | |
| 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).
If the risk ratio is < 1 then the rate of an event is decreased compared to controls. The relative risk
reduction should therefore be calculated (see below).
Relative risk reduction (RRR) or relative risk increase (RRI) is calculated by dividing the absolute
risk change by the control event rate
Using the above data, RRI = (EER – CER) / CER = (0.6 – 0.25) / 0.25 = 1.4 = 140%
