Every day, we take steps to reduce the risks in our lives. We wear seat belts to avoid getting hurt and we brush our teeth to protect them from cavities. Even so, most of us don't spend much time thinking about risk. However, it's an important concept worth exploring, especially when it comes to matters of health and science. "Risk" in these 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.
Absolute 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. It is estimated by looking at a large group of people who are similar in some way (in terms of age, for example) and counting how many people in the group develop the disease in question over a certain time period.
For example, if we followed 100,000 women ages 30 to 34 for one year, about 25 women would develop breast cancer [3]. 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. Another way to say this is that the chances of developing breast cancer in the next year are 25 in 100,000 (or 1 in 4,000) for the average 30- to 34-year-old woman.
In another example, if we followed 100,000 women ages 70 to 74 for one year, about 443 of them would develop breast cancer [3]. This means the one-year absolute risk of breast cancer for a 70- to 74-year-old woman is 443 in 100,000 women, or 1 in 226 women.
Knowing the absolute risk of a disease can help you understand the health risks in your life. In the examples above, for instance, we see the absolute risk of breast cancer is low in young women and much higher in older women.
Lifetime risk
One absolute risk we often see is the lifetime risk of breast cancer. Women in the United States have a "1 in 8” (or about 12 percent) lifetime risk of getting breast cancer [4]. 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-year risk 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.
Risk factors
Anything that affects 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).
Lack of exercise is an example of a risk factor for breast cancer. Women who are not active have a higher chance of getting breast cancer than women who are active (learn more).
Relative risk
Though the term "relative risk" may not sound familiar, you often see or hear about relative risks in news stories about health. A relative risk is a way to present the increase or decrease in risk due to a certain risk factor.
A relative risk is calculated by comparing two absolute risks. The numerator (the top number in a fraction) is the absolute risk among those 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.
Understanding relative risks
Say a study finds that women who don't exercise have a 25 percent increase in breast cancer risk compared to women who do exercise. This statistic is a relative risk. It means women who don't exercise are 25 percent more likely to develop breast cancer than women who do exercise.
It’s important to remember that relative risks depend on the underlying absolute risks of the disease. When a condition is rare (such as breast cancer among young women), a high relative risk leads to only a few extra cases. By contrast, when a condition is more common (such as breast cancer among older women), even a small relative risk can mean a lot more cases among those with the risk factor.
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. In contrast, 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. The study found 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). And, it 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).
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So, we see the relative risk is 1.25 for women who are inactive compared to those who are active. And, 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).
However, in women ages 20 to 29, the study finds 5 per 100,000 who were inactive developed breast cancer in the next year. And, 4 per 100,000 women who were active got breast cancer.
Here too, the relative risk is 1.25. However, in the younger women, being inactive caused only one extra case of breast cancer in 100,000 women. Thus, 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).
Reading relative risks
Relative risks are presented in many ways. This brief guide may help you recognize a relative risk when you hear it on the news or read it on the Internet.
When a relative risk is between 1 and 1.99 it may be presented in several ways. Say, from the example of inactivity above, the relative risk is 1.25. You may see:
- "Inactivity has a relative risk of 1.25."
- "Inactivity increases the risk of breast cancer by 25 percent."
- "Inactivity increases risk by 1.25-fold."
When a relative risk is 2 or more, it is often presented as how many times the risk is increased. For example, atypical hyperplasia, a benign breast condition, has a relative risk of about 4. You may see:
- "Atypical hyperplasia has a relative risk of 4.”
- “Women with atypical hyperplasia have 4 times the risk those without atypical hyperplasia."
- "There is a 4-fold increase in risk among women with atypical hyperplasia."
When a relative risk is below 1, it means that the risk factor lowers the risk of disease. The relative risk of breast cancer for prolonged breastfeeding (a lifetime total of one year or more) is about 0.75. You may see:
- “Breastfeeding has a relative risk of 0.75.”
- "Breastfeeding lowers risk by 25 percent."
Making healthy choices
A healthy lifestyle is always important, no matter your underlying risk of breast cancer. However, understanding how absolute risk and relative risk are related can help you be a well-informed consumer of health information. You can also use this knowledge to make informed health choices in your life.
Learn more about healthy behaviors and breast cancer risk.
Updated 01/19/12
