UNDERSTANDING RISK

Understanding Risk

Day in and day out, we all take steps to manage 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. Yet, even while we take these types of steps every day, most of us don’t spend much time thinking about risk. But it’s actually an important concept that’s worth exploring, especially when it comes to matters of health and science. “Risk” in these fields can have a number of different meanings, and knowing some of the basic types of risk can help you better understand your chances of developing a disease like breast cancer and the steps you can take to protect yourself from it.

Absolute Risk
The most basic type of risk is absolute risk, which is simply a person's chance of developing a specific disease over a certain time period. 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 the number of people in the group who develop the disease in question over the specified time period.

For example, if we followed 100,000 women between the ages of 20 and 29 for one year, approximately 4 would develop breast cancer during this period. 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, approximately 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 disease can help women prioritize the health risks in their lives. 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 that many women are familiar with is the lifetime risk of breast cancer. Currently, women in the US have a “1 in 8”, or approximately 12 percent, lifetime risk of developing breast cancer [2]. This statistic 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. Not surprisingly, the lifetime risk of breast cancer is much higher than the one-year risk of breast cancer. This is because the lifetime risk is a type of summing, or adding up, of all the one-year absolute risks over a woman's life span, up to age 85.

Risk Factors
Anything that influences a person’s absolute risk of developing a disease is called a risk factor. A risk factor can be anything from a lifestyle choice (such as diet) to a genetic component (such as family history) to an environmental exposure (such as radiation). For instance, lack of exercise is a risk factor for breast cancer. Women who are not regularly active have a higher risk of breast cancer than women who are regularly active.

Relative Risk
Though the term “relative risk” may not sound familiar, you likely see or hear about relative risks all the time in news stories about health. A relative risk is a generalized way to present the increase or decrease in risk that’s due to a particular risk factor.

A relative risk is calculated by taking the ratio of two absolute risks: the numerator (the top number in the ratio) is the absolute risk among those with the risk factor, while 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.




Say there is a new study published that finds that women who don’t exercise regularly have a 25 percent increase in the risk of breast cancer compared to women who do exercise regularly: 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.

Understanding relative risks
What’s important to realize about relative risks is that they depend on the underlying absolute risks. Overall, when a disease is rare, as breast cancer is among very young women, a relative risk that sounds high will still mean that only a few extra cases of disease will arise. By contrast, when a disease is more common, as breast cancer is among older women, even small relative risks can mean a lot more cases of disease.

One way to think about this is in terms of money. If a person has a single dollar, this makes dollars "rare". If this person doubles her money, she only gains one extra dollar. In contrast, if a person has a million dollars, this makes dollars "common", and a doubling of her money means she gains a million extra dollars. In both cases, there is a doubling of money, but the real increase in dollars is very 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 develop for a given relative risk.

Our previous example of the exercise study can show this as well. In this scenario, the study found that physically inactive women have a 25 percent greater risk of breast cancer than active women do. Since older women are more likely to develop breast cancer than younger women, the potential impact that lack of physical activity has on breast cancer is greater in older women than in younger women. And the numbers below show this.

Among women aged 70-74, 500 women per 100,000 who are physically inactive could develop breast cancer in the next year, while 400 women per 100,000 who are active could develop the disease.




So, among older women aged 70-74, being inactive could potentially lead to 100 more cases of breast cancer for every 100,000 women.

Among women aged 20-29, however, being inactive would cause only 1 extra case of breast cancer in 100,000 women. Among these younger women, 4 women per 100,000 who are active could get breast cancer in the next year, while 5 women per 100,000 who are inactive could get breast cancer.




Of course, a healthy lifestyle should be maintained throughout life, not just when one’s underlying risk of disease crosses a certain threshold. But, understanding how absolute risk and relative risk interrelate can help women become better informed consumers of health information and use this knowledge to make informed decisions about the health choices in their lives.

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 the newspaper, hear it on the TV news or read it in on the Internet.

When a relative risk is between 1.0 and 1.99, as physical inactivity is, it might be presented in any of the following ways:

• “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 in relation to how many times the risk is increased. If, for example, a factor has a relative risk of 3.0, it can also be stated as follows:

• "Risk is increased 3 times."

• "There is a 3-fold increase in risk."

When a relative risk is below 1.0, it means that the risk factor actually lowers the risk of disease. If, for example, a factor lowered the risk of breast cancer by 25 percent, it could also be stated as follows:

• “The factor has a relative risk of 0.75.”

• “This factor lowers risk by 25 percent.”