The Who, What, Where, When and Sometimes, Why.

Understanding Risk

There are different types of 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 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.

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.

Example of 1-year absolute risks

If we followed 100,000 women ages 30-34 for one year, about 29 women would develop breast cancer [607].

So, the 1-year absolute risk of breast cancer for a 30-34 year-old woman is 29 per 100,000 women (or 1 per 3,448 women). This risk is less than 1 percent.

This means the chances of getting breast cancer in the next year are less than 1 percent for the average 30-34 year-old woman.

This example shows the 1-year absolute risk of breast cancer for a young woman is low.

Examples of 10-year absolute risks

You may see absolute risk presented over long periods of time.

The table below shows the 10-year absolute risks of breast cancer by age. It shows the 10-year absolute risk of breast cancer is low in young women and higher in older women.

 Absolute risk of breast cancer in U.S. women by age

If current age is:

Absolute risk of developing breast cancer in the next 10 years is:

20

1 in 1,429 (0.07%)

30

1 in 208 (0.5%)

40

1 in 63 (1.6%)

50

1 in 42 (2.4%)

60

1 in 28 (3.5%)

70

1 in 25 (4.1%)

80

1 in 33 (3.0%)

 

Lifetime risk

1 in 8 (12.9%)

Source: SEER Cancer Statistics Review, 1975-2017, 2020 [608]

 

Lifetime risk

One absolute risk you may see is lifetime risk of breast cancer. Women in the U.S. have a 1 in 8 (or about 13 percent) lifetime risk of getting breast cancer [608].

This means for every 8 women in the U.S., 1 will be diagnosed with breast cancer during her lifetime.

Lifetime risk of breast cancer adds up all the 1-year absolute risks over a woman’s life. So, it’s much higher than the 1- or 10-year absolute risks of breast cancer.

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 that increases or decreases a person’s absolute risk 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 period)
  • The environment (such as radiation exposure)

Some factors increase 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 decrease risk. For example, women who breastfeed have a lower risk of breast cancer than women who don’t. So, breastfeeding 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 the risk in people without the factor.

A relative risk compares 2 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 the relative risk.

  

When relative risk is:

This shows:

Greater than 1
(for example, 1.5 or 2.0)

People with the risk factor have a higher risk than people without the risk factor.

A relative risk of 1.5 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, 0.8)

People with the risk factor have a lower risk than people without the risk factor.

A relative risk of 0.8 means someone with the risk factor has a 20 percent lower risk of breast cancer than someone 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 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.

Understanding relative risk

The importance of absolute risk to relative risk

The impact of a relative risk depends on the underlying absolute risk of a disease.

  • 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 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, 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 during 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. 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 70-74, 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 during 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) was different depending on the underlying absolute risk.

Reading relative risks

Relative risks can be presented in many ways. This guide may help you understand a relative risk when you see or hear it.

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 percent 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.”
  • “There’s a 4-fold increase in the risk of breast cancer among women with atypical hyperplasia 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 percent lower risk of breast cancer compared to women who do not breastfeed.”

Relative risks in research

You can put your knowledge of relative risks to work right away.

Our Breast Cancer Research Studies section has summary tables of research on topics ranging from risk factors to treatment to social support.

These tables show research behind many recommendations and standards of care discussed in this section.

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.

 

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