Every day, we take steps to reduce the risks in our lives. We wear seat belts to reduce the chance that we’ll get hurt in a car accident and we brush our teeth to prevent cavities, but most of us don't spend too much time thinking about risk. However, it's important to understand risk as it relates to health.
"Risk" in the health and medical fields can have special meanings. Knowing the basic types of risk can help you understand your chances of getting breast cancer and the steps you can take to lower your risk.
The most basic type of risk is absolute risk. Absolute risk is a person's chance of developing a certain disease over a certain period of time.
Absolute risk is estimated by looking at a large group of people who are similar in some way (the same age, for example) and counting how many people in the group develop a certain disease over a certain period of time. Knowing the absolute risk of a disease can help you understand the health risks in your life.
The examples below show the absolute risk of breast cancer is low in young women and much higher in older women.
For example, if we followed 100,000 women ages 30 to 34 for one year, about 25 women would develop breast cancer . This means the one-year absolute risk of breast cancer for a 30- to 34-year-old woman is 25 per 100,000 women (or 1 per 4,000 women) or a risk of less than one percent. Another way to say this is that the chances of developing breast cancer in the next year are less than one percent for the average 30- to 34-year-old woman.
From the examples above, we see that the absolute one-year risk of breast cancer for a woman is low.
You may see absolute risk presented over longer periods of time. The table below shows the 10-year absolute risks of breast cancer by age.
Absolute risk of breast cancer in American women by age
If current age is:
Absolute risk of developing breast cancer in the next 10 years is:
1 in 1,732 (0.06%)
1 in 228 (0.4%)
1 in 69 (1.5%)
1 in 43 (2.3%)
1 in 29 (3.5%)
1 in 26 (3.8%)
1 in 8 (12.3%)
Source: American Cancer Society, Breast Cancer Facts & Figures 2013-2014 .
One absolute risk you may see is the lifetime risk of breast cancer. Women in the U.S. have a "1 in 8” (or about 12 percent) lifetime risk of getting breast cancer [4-5]. This means that for every eight women in the U.S. who live to be age 85, one will be diagnosed with breast cancer during her lifetime.
The lifetime risk of breast cancer is much higher than the one- or 10-year absolute risks of breast cancer. This is because the lifetime risk adds up all the one-year absolute risks over a woman's life span, up to age 85.
Learn how lifetime risk of breast cancer varies worldwide.
Anything that increases or decreases a person's absolute risk of developing a disease is called a risk factor. A risk factor can be related to lifestyle (such as lack of exercise), genetics (such as family history) or the environment (such as radiation exposure).
Some factors increase risk. For example, older women have a higher risk of getting breast cancer than younger women. So, age is a risk factor for breast cancer. Some factors decrease risk. For example, women who breastfeed have a lower risk of getting breast cancer than women who do not (learn more). So, breastfeeding is also a risk factor for breast cancer.
A relative risk shows how much higher, how much lower or whether there is no difference in risk in people with a certain risk factor compared to the risk in people without the factor.
A relative risk compares two absolute risks. The numerator (the top number in a fraction) is the absolute risk among people with the risk factor. The denominator (the bottom number) is the absolute risk among those without the risk factor. The absolute risk of those with the factor divided by the absolute risk of those without the factor gives you the relative risk.
When relative risk is:
Greater than 1(for example, 1.5 or 2.0)
People with the risk factor have a higher risk compared to people without the risk factor.
A relative risk of 1.50 means someone with the risk factor has a 50 percent higher risk of breast cancer than someone without the factor. A relative risk of 2.0 means someone with the risk factor has twice the risk (or 2-fold the risk) of someone without the factor.
Less than 1(for example, 0.80)
People with the risk factor have a lower risk compared to people without the risk factor.
A relative risk of 0.80 means someone with the risk factor has a 20 percent lower risk of breast cancer than someone without the factor.
A relative risk of 1 means there is no difference in risk between people with and without the risk factor.
Say a study shows that women who don't exercise (inactive women) have a 25 percent increase in breast cancer risk compared to women who do exercise (active women). This statistic is a relative risk (the relative risk is 1.25). It means inactive women are 25 percent more likely to develop breast cancer than women who exercise.
The impact of a relative risk depends on the underlying absolute risk of a disease.
We can think about relative risk in terms of money. If you have a single dollar, this makes dollars "rare.” If you double your money, you only gain one extra dollar. But, if you have a million dollars, this makes dollars "common" and a doubling your money means you gain a million extra dollars. In both cases, you double your money, but the real increase in dollars is quite different. The same is true with disease risk. The higher the absolute risk of getting a disease, the greater the number of extra cases that will occur for a given relative risk.
Using our example of the exercise study, we can also show how absolute risks affect the number of extra cases. Inactive women have a 25 percent greater risk of breast cancer than active women (a relative risk of 1.25). Since older women are more likely to get breast cancer, lack of exercise has a greater impact on breast cancer risk in older women than in younger women.
Let’s first look at the women in the study ages 70 to 74 years. The study finds that 500 women per 100,000 who are inactive develop breast cancer during one year (this is the absolute risk for women with the risk factor, lack of exercise). The study also shows that 400 women per 100,000 who are active develop breast cancer (this is the absolute risk for women without the risk factor).
So, the relative risk is 1.25 for women who are inactive compared to those who are active.
Among women ages 70 to 74, being inactive led to 100 more cases of breast cancer per 100,000 women in one year (500 cases – 400 cases = 100 cases).
Now let’s look at the women in the study ages 20 to 29. The study finds that 5 per 100,000 who were inactive developed breast cancer in the next year. And, 4 women per 100,000 who were active got breast cancer.
Here, the relative risk is also 1.25. However, in women ages 20 to 29, being inactive led to only one extra case of breast cancer per 100,000 women (5 cases – 4 cases = 1 case).
So, the same relative risk of 1.25 led to many more extra cases of breast cancer in the older women (100 extra cases) than in the younger women (one extra case). The impact of the same relative risk (1.25) was different depending on the underlying absolute risk.
Relative risks can be presented in many ways. This guide may help you recognize a relative risk when you see or hear it.
When a relative risk is between 1 and 1.99 it may be presented in several ways. For example, in the exercise study above, the relative risk is 1.25. You may see:
When a relative risk is 2 or more, it is often presented as how many times the risk is increased. For example, women with atypical hyperplasia, a benign breast condition, have a relative risk of about 4 compared to women without atypical hyperplasia. You may see:
When a relative risk is less than 1, it means that the risk factor lowers the risk of disease. For example, women who breastfeed (for a lifetime total of one year or more) have a relative risk of breast cancer of about 0.75 compared to women who do not breastfeed. You may see:
You can put your understanding of relative risks to work right away. Our Breast Cancer Research section has summary tables of the current body of research on the topics ranging from risk factors to treatment to social support. These tables show some of the research behind many of the recommendations and standards of practice discussed throughout Understanding Breast Cancer.
If you’re not familiar with how the research process works (or just need a refresher), “How to read a research table” is a good place to start before looking at the tables.
Learn more about breast cancer research.
Understanding absolute risk and relative risk can help you be a well-informed consumer of health information. You can also use this knowledge to make informed choices about your health.
No matter what your underlying risk of breast cancer is, a healthy lifestyle is always important. Learn more about healthy behaviors and breast cancer risk.
Risk Factors and Risk Reduction Introduction