Every day, we take steps to reduce the risks in our lives. We wear our seatbelts to avoid getting hurt in a car crash. We brush our teeth to protect them from cavities and we carry umbrellas to cut down on the risk of getting soaked in a storm. Even so, most of us don't spend much time thinking about risk. However, it's a concept worth exploring, especially when it comes to matters of health and science. "Risk" in these fields can have different 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, which is a person's chance of developing a specific disease over a certain period of time. Absolute risk is estimated by looking at a large group of people who are similar in some respect (in terms of age, for example) and counting how many people in the group develop the disease in question over a specified time period.
For example, if we followed 100,000 women between the ages of 20 and 29 for one year, about 4 would develop breast cancer. This means that the one-year absolute risk of breast cancer for a 20- to 29-year-old woman is 4 per 100,000 women, or 1 per 25,000 women. Another way to say this is that the chances of developing breast cancer in the next year are 4 in 100,000 (or 1 in 25,000) for the average 20- to 29-year-old woman.
In another example, if we followed 100,000 women aged 70 to 74 for a one-year period, about 400 of them would develop breast cancer. This means the one-year absolute risk of breast cancer for a 70- to 74-year-old woman is 400 per 100,000 women, or 1 per 250 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 that the absolute risk of breast cancer is low in young women and much higher in older women.
Lifetime risk
One absolute risk you may hear often is the lifetime risk of breast cancer. Currently, women in the U.S. have a "1 in 8”, or about 12 percent, lifetime risk of breast cancer [3]. This means that for every 8 American women who live to be age 85, one of them 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.
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 your lifestyle (such as diet), genetics (such as family history) or an environmental exposure (such as radiation). For instance, lack of exercise is a risk factor for breast cancer. Women who are not active have a higher chance of getting breast cancer than women who are active.
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 that's 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. When the absolute risk of those with the factor is divided by the absolute risk of those without the factor, the number you get is 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 that 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. When a condition is rare (such as breast cancer among young women), a high relative risk still means that only a few extra cases will arise. By contrast, when a condition is more common (such as breast cancer among older women), even small relative risks can mean a lot more cases.
One way to think about this is 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.
Our example of the exercise study can show this as well. The study found that inactive women have a 25 percent greater risk of breast cancer than active women. 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. For example, say a study looks at women aged 70-74 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, say this 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). Here, the relative risk is 1.25 for women who are inactive compared to those who are active.
So, among older women aged 70-74, being inactive may lead to 100 more cases of breast cancer per 100,000 women in one year.
In women aged 20-29, however, being inactive would cause only one extra case of breast cancer in 100,000 women. Among these younger women, 5 per 100,000 who are inactive could get breast cancer in the next year, while 4 per 100,000 who are active could get breast cancer.
A healthy lifestyle is always important, no matter your underlying risk of disease. 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.
Reading relative risks
Relative risks are presented in many ways. Here's a brief guide to help you recognize a relative risk when you see it in a newspaper, hear it on the news or read it on the Internet.
When a relative risk is between 1.0 and 1.99 it may be presented in several ways. Suppose, 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 2.0 or above, 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 4.0. This can be stated as:
- "Atypical hyperplasia has a relative risk of 4.0.”
- “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 1.0, 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."
Updated 11/09/09
