The odds ratio is defined as the ratio of the odds of A in the presence of B and the odds of A in the presence of B.

The odds ratio can also be used to determine whether a particular exposure is a risk factor for a particular outcome, and to compare the magnitude of various risk factors for that outcome.

**Formula to calculate odds ratio.**

**Example:**

If 12 smokers have lung cancer, 60 smokers do not have lung cancer, 1 non-smoker has lung cancer, and 90 non-smokers do not have lung cancer, calculate the odd ratio.

Thus, the smokers has 20 times the odds of having lung cancer than non-smokers.