To help explain absolute and relative risk, here are some examples using a washing machine.
Suppose you bought a new washing machine. The manual tells you that your new machine is covered by a warranty for the first year. The manual also says that on average, 10% of the machines will need service during the first year. So the absolute risk of your machine needing service in the first year is 10%.
The manual also says that certain things that you might do will increase or decrease the risk of needing service in the first year.
The risk of needing service in the first year decreases to 8% if you use powder laundry detergent instead of a liquid laundry detergent.
The risk of needing service in the first year increases to 15% if you use bleach compared to never using bleach.
Now let's look at the relative risk, absolute risk, and hazard ratio for powder-only users and bleach users.
Powder detergent-only users
The absolute risk of needing service in the first year decreases by 2% if you use only powder detergent. 2% is the difference between the initial 10% absolute risk of needing service and the 8% absolute risk if you use only powder detergent (10% - 8% = 2%). You can say that only using powder detergent lowers the absolute risk by 2%.
The 2% drop in absolute risk is a 20% relative risk decrease because you divide 2% by 10% (.02 ÷ .10 = .20, or 20%). In other words, relative to the 10% absolute risk, the 8% absolute risk is 20% lower. You can say that only using powder detergent results in a 20% reduction in relative risk. Even though the relative risk decreases 20% if you use only powder detergent, the absolute risk decreased by only 2% (8% vs. 10%).
Because 8% (powder-only absolute risk) divided by 10% (absolute risk) equals 0.8, the hazard ratio is 0.8.
The absolute risk of needing service in the first year increases by 5% if you use bleach. 5% is the difference between the 10% absolute risk of needing service and the 15% absolute risk if you use bleach (15% - 10% = 5%). You can say that using bleach increases the absolute risk by 5%.
The 5% increase in absolute risk -- from 10% to 15% -- is a 50% relative risk increase because you divide 5% by 10% (.05 ÷ .10 = .50, or 50%). In other words, relative to the 10% absolute risk, the 15% absolute risk is 50% higher. You can say that using bleach results in a 50% increase in relative risk. Even though the relative risk increases 50% if you use bleach, the absolute risk increases only 5% (15% vs. 10%).
Because 15% (bleach-using absolute risk) divided by 10% (absolute risk) equals 1.5, the hazard ratio is 1.5.
When you read about research on treatments or lifestyle choices that can reduce or increase breast cancer risk, it's important to know that in most cases the change in risk is for a specific time period that's noted in the study. Also, when you read that something caused an increase or decrease in breast cancer risk, this change in risk is usually relative risk unless it's otherwise noted.
Knowing how much your breast cancer risk changes with lifestyle choices and treatment options can help you and your doctor make the best decisions for YOU.
Learn about Breast Cancer Risk Factors and choices you can make to lower your risk.
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