A. 0.8 – Explanation
| Odds – remember a ratio of the number of people who incur a particular outcome to the number of people who do not incur the outcome NOT a ratio of the number of people who incur a particular outcome to the total number of people |
Odds of MI in group B = 10/50 = 1/5
Odds of MI in group A = 20/80 = 1/4
Odds ratio of having a MI = 1/5 divided by 1/4 = 0.8
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 dysmenorrhea 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
