Suppose that 500 patients are treated with drug A and 500 with placebo.

The outcome is survival for 30 days. In the treatment arm 85% of the patients survive 30 days, and in the placebo arm 70% of the patients survive 30 days. The two-by-two table looks as above.The odds ratio is calculated as the ratio of the odds of treatment in the patients who survived (425/350 = 1.21) and the odds of treatment in the patient who did not survive (75/150 = 0.50), resulting in an odds ratio of 1.21/0.50 = 2.43. One could also calculate the cross-product of the table: (425*150)/(350*75) = 2.43.The risk ratio is calculated as the ratio of the risk of survival in the treatment group (425/500 = 0.85) and the risk of survival in the placebo group (350/500 = 0.70), resulting in a risk ratio of 0.85/0.70 = 1.21.