Understanding Risk
There are different ways of talking about risk in the health field. Knowing the basic types of risk can help you understand your chances of getting breast cancer and the steps you can take that may be linked to a lower 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.
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 your risk of the disease.
Example of a 1-year absolute risk
If we followed 100,000 women ages 30-34 for one year, about 30 women would develop breast cancer [2].
So, the 1-year absolute risk of breast cancer for a 30-34-year-old woman is about 30 per 100,000 women (or 1 per 3,333 women). This risk is less than 1%.
This means the chances of getting breast cancer in the next year are less than 1% for the average 30-34-year-old woman.
This example shows the 1-year absolute risk of breast cancer for young women is low.
Examples of 10-year absolute risks
A 10-year absolute risk of breast cancer shows a person’s chance of getting breast cancer in the next 10 years. As with 1-year absolute risks, 10-year absolute risks vary by age. The 10-year absolute breast cancer risk is lower for younger women than for older women.
For example, the 10-year absolute risk for a 30-year-old woman is about 1 in 200 (less than 1%), and the 10-year absolute risk for a 70-year-old woman is about 1 in 24 (about 4%) [3-4].
This means the chances of getting breast cancer in the next 10 years are less than 1% for a 30-year-old woman and about 4% for a 70-year-old woman.
You may see absolute risk presented over longer periods of time, such as lifetime risk.
Lifetime risk
Lifetime risk of breast cancer is an absolute risk. Women in the U.S. have a 1 in 8 (or about 13%) lifetime risk of getting breast cancer [643].
This means for every 8 women in the U.S., 1 will be diagnosed with breast cancer during her lifetime. This doesn’t mean 1 in 8 women you know will get breast cancer. It’s more that on average, in a group of 8 women of varying ages, 1 in 8 would be expected to be diagnosed with breast cancer in her lifetime.
Lifetime risk is estimated from data on shorter-term absolute risks. So, there are limits to how well it reflects true breast cancer risk over a lifetime.
Learn how the lifetime risk of breast cancer varies by race/ethnicity.
Learn how the lifetime risk of breast cancer varies worldwide.
Risk factors
Anything linked to a person’s chance of getting a disease is called a risk factor.
A risk factor can be related to:
- Genetics (such as family history)
- Lifestyle (such as exercise)
- Reproduction (such as age at your first menstrual period)
- The environment (such as radiation exposure)
Some factors are linked to an increased risk. For example, older women have a higher risk of breast cancer than younger women. So, age is a risk factor for breast cancer.
Some factors are linked to a decreased risk. For example, women who exercise have a lower risk of breast cancer than women who don’t exercise. So, exercise is also a risk factor for breast cancer.
Relative risk
A relative risk shows how much higher, how much lower or whether there’s no difference in risk for people with a certain risk factor compared to people without the factor.
A relative risk compares 2 absolute risks and is calculated in this way:
- 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 people with the factor divided by the absolute risk of those without the factor gives the relative risk.
|
When relative risk is: | This shows: |
Greater than 1 | People with the risk factor have a higher risk than people without the risk factor. A relative risk of 1.5 means people with the risk factor have a 50% higher risk of breast cancer than those without the factor. A relative risk of 2.0 means people with the risk factor have twice the risk (or 2-fold higher risk) of those without the factor. |
Less than 1 |
People with the risk factor have a lower risk than people without the risk factor. A relative risk of 0.8 means people with the risk factor have a 20% lower risk of breast cancer than those without the factor. |
1 |
A relative risk of 1 means there’s no difference in risk between people with the factor and people without the factor. |
Example of a relative risk
Say a study shows women who don’t exercise (inactive women) have a 25% higher 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% more likely to develop breast cancer than women who exercise.
Understanding relative risk
The importance of absolute risk to understand the impact of relative risk
The impact of a relative risk depends on the underlying absolute risk of a disease (a person’s chance of developing the disease over a certain period of time).
- When a disease is rare (such as breast cancer among young women), a high relative risk leads to only a few extra cases among those with the risk factor.
- When a disease is more common (such as breast cancer among older women), even a small relative risk can lead to many more cases among those with the risk factor.
We can think about relative risk in terms of money.
If you only have one dollar, this makes dollars “rare.” If you double your money, you only gain one extra dollar.
But, if you have one million dollars, this makes dollars “common” and doubling your money means you gain one million extra dollars.
In both cases, you double your money, but the 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.
Example of the impact of a relative risk
Using our example of the exercise study above, we can show how absolute risks affect the number of extra cases.
Inactive women have a 25% higher risk of breast cancer than active women (a relative risk of 1.25).
Since older women are more likely to get breast cancer, a lack of exercise has a greater impact on breast cancer risk in older women than in younger women.
First, let’s look at the women in the study ages 70-74 years.
The study finds 500 women per 100,000 who are inactive develop breast cancer in one year. This is the absolute risk for women with the risk factor, lack of exercise.
The study also shows 400 women per 100,000 who are active develop breast cancer in one year. This is the absolute risk for women without the risk factor.
|
The relative risk is 1.25 for women who are inactive compared to those who are active.
Among women ages 70-74, 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 20-29.
The study finds 5 women per 100,000 who were inactive developed breast cancer in one year. And 4 women per 100,000 who were active got breast cancer.
|
Here again, the relative risk is 1.25.
However, in women ages 20-29, being inactive led to only 1 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 (1 extra case).
The impact of the same relative risk (1.25) depended on the underlying absolute risk.
Reading relative risks
Relative risks can be presented in many ways. The examples below may help you understand a relative risk when you see or hear it in the news or other media report.
Relative risks greater than 1
A relative risk between 1 and 1.99 may be presented in several ways.
For example, in the exercise study above, the relative risk was 1.25.
You may see:
- Inactive women have a relative risk of 1.25 compared to active women.
- Inactive women have a 25% higher risk of breast cancer compared to active women.
- Inactive women have a 1.25-fold increased risk of breast cancer compared to active women.
When a relative risk is 2 or more, it’s often presented as the number of 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:
- Women with atypical hyperplasia have 4 times the risk of breast cancer of women without atypical hyperplasia.
- Women with atypical hyperplasia have a 4-fold increased risk of breast cancer compared to women without atypical hyperplasia.
Relative risks less than 1
A relative risk less than 1 means the risk factor lowers the risk of disease.
For example, women who breastfeed for a year have a relative risk of breast cancer of about 0.94 compared to women who don’t breastfeed.
You may see:
- Women who breastfeed have a 6% lower risk of breast cancer compared to women who don’t breastfeed.
Relative risks in research
Research studies often report findings as relative risks. You will see relative risks in the research summary tables in our Breast Cancer Research Studies section. These tables cover topics ranging from risk factors to treatment to social support. They show the research behind many recommendations and standards of care related to breast cancer.
Reading research tables
If you don’t know how the research process works (or just need a refresher), our How to read a research table section is a good place to start before looking at the tables.
Learn more about breast cancer research.
Making healthy lifestyle choices
Understanding absolute risk and relative risk can help you make informed choices about your health.
No matter your risk of breast cancer, a healthy lifestyle is important.
Learn more about a healthy lifestyle and breast cancer risk.
Learn about a healthy lifestyle for people who’ve had breast cancer.
Updated 07/15/24