C. The odds ratio approximates to relative risk if the outcome of interest is rare – Explanation
Odds and odds ratio
Odds are a ratio of the number of people who incur a particular outcome to the number of people
who do not incur the outcome. The odds ratio may be defined as the ratio of the odds of a particular
outcome with experimental treatment and that of control. Odds ratios are the usual reported measure in case-control studies. It approximates to relative risk if
the outcome of interest is rare. 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 | Achieved = 50% pain relief | |
| Paracetamol | 60 | 40 |
| Placebo | 90 | 30 |
The odds of achieving significant pain relief with paracetamol = 40 / 20 = 2
The odds of achieving significant pain relief with placebo = 30 / 60 = 0.5
Therefore the odds ratio = 2 / 0.5 = 4
