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Absolute vs. Relative Risk: What Does Percentage Risk Really Mean?


Knowing that limiting how much alcohol you drink or exercising regularly can decrease your breast cancer risk is important. But you probably want to know just how much taking those steps -- limiting alcohol and exercising regularly -- can lower your risk.

Similarly, if you've been diagnosed and your doctor tells you that a certain treatment can reduce your risk of recurrence (the cancer coming back) by 40%, you probably want to know what that really means for you.

Understanding the terms relative risk and absolute risk can help you better understand your own risk of breast cancer.

Relative risk is the number that tells you how much something you do, such as maintaining a healthy weight, can change your risk compared to your risk if you're very overweight. Relative risk can be expressed as a percentage decrease or a percentage increase. If something you do or take doesn't change your risk, then the relative risk reduction is 0% (no difference). If something you do or take lowers your risk by 30% compared to someone who doesn't take the same step, then that action reduces your relative risk by 30%. If something you do triples your risk, then your relative risk increases 300%.

Absolute risk is the size of your own risk. Absolute risk reduction is the number of percentage points your own risk goes down if you do something protective, such as stop drinking alcohol. The size of your absolute risk reduction depends on what your risk is to begin with.

Hazard Ratios. Doctors sometimes use the term "hazard ratio" to talk about risk. A hazard ratio considers your absolute risk to be 1. If something you do or take doesn't change your risk, then the hazard ratio is 1. If something you do or take lowers your risk by 30% compared to someone who doesn’t take the same step, then that action makes your hazard ratio 0.70, which means that the risk is 70% of what it was without taking the step (in other words, it's 30% lower). If something you do triples your risk, then your hazard ratio is 3.0 (your risk is 3 times greater than it was before you did the thing that increased your risk).

On the next page, you can read about Examples of Risk Increasing and Decreasing.

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