C. 2 – Explanation
The odds of developing hypertension if you work < 40 hours/week is 50 / 1000 = 1 / 20 or 0.05
The odds of developing hypertension if you work > 40 hours/week is 60 / 600 = 1 / 10 or 0.1
The odds ratio is therefore = (odds of developing hypertension if you work > 40 hours/week) / (odds
of developing hypertension if you work < 40 hours/week) = 0.1/0.05 = 2
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 placebo = 30 / 60 = 0.5
Therefore the odds ratio = 2 / 0.5 = 4
