Odds ratio is calculated to identify the likelihood of exposure to a risk when comparing two groups, one with and one without disease.

It is calculated by dividing the odds of exposure in the disease group by odds of exposure in the non-disease group.

A ratio of 1 means no association; greater than 1 indicates an association between exposure and disease

For example,

If the prevalence of smoking among lung cancer patients is 95 per 100, and the prevalence of smoking among people without lung cancer is 25 per 100, then

Odds Ratio =95/ 25 = 3.8