We often hear news reports that something has been found to reduce the risk of breast cancer, or reduce the risk of dying from breast cancer. These reports are often misleading because they use numbers representing relative risk reductions. Absolute risk reduction is what really matters, which is often much lower, and often not reported. What is the difference?
Relative risk is used to compare risks between two groups, whereas absolute risk stands on its own. Here is an example: Say there is a clinical trial evaluating a new drug that will prevent breast cancer, and 200 women have signed up. In the control group, 100 women received a placebo pill and two developed breast cancer. In the treatment group, 100 people received the drug and only one person developed breast cancer. The two groups are compared – two developed breast cancer in the control group vs one in the treatment group. A 50% reduction in breast cancer! That sounds pretty good. People who want to avoid breast cancer might consider taking this drug, even if there are side effects. But the reality is that the absolute risk reduction was much smaller. If the risk of developing breast cancer at all was 2%, taking the drug may lower the risk to 1%. So a 1% change in absolute risk of breast cancer might not seem worth it if there are side effects to the drug.