060813 Absolute and Relative Risk Absolute Risk The

06/08/13 Absolute and Relative Risk Absolute Risk - The chance/probability of someone in a group experiencing an outcome. Relative Risk - How many times more/less likely one group is of experiencing an outcome compared to another group. The group you are "comparing to" should always be on the bottom of the fraction.

Example: Physicians Health Study • 22000 physicians randomly assigned to aspirin or placebo, then wait eight years Treatment Heart attack No heart attack Total aspirin 104 10933 11037 placebo 189 10845 11034 total 293 21778 22071 (a) What is the absolute risk of someone in the aspirin group having a heart attack? Risk in aspirin group = 104/11037 = 0. 0094 (b) What is the absolute risk of someone in the placebo group having a heart attack? Risk in placebo group = 189/11034 = 0. 0171

Example: Physicians Health Study • 22000 physicians randomly assigned to aspirin or placebo, then wait eight years Treatment Heart attack No heart attack Total aspirin 104 10933 11037 placebo 189 10845 11034 total 293 21778 22071 (c) What is the relative risk of someone in the aspirin group having a heart attack compared to the placebo group? Relative Risk = 0. 0094/0. 0171 = 0. 550 (d) What is the relative risk of someone in the placebo group having a heart attack compared to the aspirin group? Relative Risk = 0. 0171/0. 0094 = 1. 819

What does it all mean? The answer from (c) tells us that your are 0. 550 times less likely to have a heart attack if you are in the aspirin group compared to the placebo group. The answer from (d) tells us that your are 1. 819 times more likely to have a heart attack if you are in the placebo group compared to the aspirin group. It is desirable to have relative risks which are greater than 1 as it easier to interpret what they mean.