Dr. Susan Love Research Foundation | Ending Breast Cancer

Relative risk is a statistical method that is used to compare the difference in results between two groups that, as a whole, researchers believe are pretty much identical in terms of age, weight, health histories, etc., but in which one group has a possible risk factor. For example, researchers could find a group of women who are pretty much the same in terms of age, race, occupation, and place of residence, and then divide them up based on which women had a mother who had breast cancer and which ones didn't. The researchers could then look to see how many women in both groups went on to develop breast cancer. The difference in risk between the two groups is the relative risk of getting breast cancer for a woman whose mother has had breast cancer.

What is important to keep in mind is that relative risk is an imprecise statistic that only describes an association, not causation. Oftentimes the risk difference between the two groups sounds large when in the fact the difference in outcome between the two groups is actually very small. In this way, relative risk can be a misleading statistic. For example, if 2 percent of the women in the first group and 4 percent of those in the second group develop breast cancer, those in the second group would be said to have 2.0 times the risk of the first group. Another way this could be stated is that the risk factor increases the risk of breast cancer by 100 percent.

This does not mean that the women in the second group have a 100 percent chance of developing breast cancer. Rather, it means their risk is 100 percent higher when the number of breast cancers seen in each group is compared—the difference between a 2 percent risk and a 4 percent risk. Hearing that a risk factor increases risk by 100 percent can be alarming. And it can easily lead you to forget that you need to put any such number in context. In the previous example the risk sounded large—100 percent—yet, in fact, the vast majority of women—98 percent of the women in the first group and 96 percent of the women in the second group—didn't develop breast cancer.

When you look at a statistic that describes relative risk, you should look and see how many women were in the study. A study that is comparing two groups of 30 women is a lot less significant than a study that is comparing two groups of 3,000 women. This is because studies of small groups of women are more likely to find an association that might actually just be due to chance. The larger the groups, the less likely the association between a risk factor and breast cancer is due to chance.

It also is important to keep in mind that relative risk refers to a population of women, not to an individual woman. For example, if a woman is diagnosed with atypical ductal hyperplasia (ADH), it means she is part of a group of women who are at increased risk for developing breast cancer, but we can't say what her specific risk is.

To better understand this, picture two ballrooms. One contains 1,000 women who had completely normal biopsies and the other contains 1,000 women recently diagnosed with ADH. Now, imagine us going back to those ballrooms 20 years later to see who developed breast cancer. We would predict that there might be a few more women who developed breast cancer in the room that contained women with a previous diagnosis of ADH. But in both rooms the vast majority of the women would not have developed breast cancer. More importantly, we would not have been able to say at the beginning which of the women were going to get breast cancer, and we could not say why it happened to them and not the others. So, while the women with ADH may have had a higher relative risk of developing breast cancer, relative risk by no means predicts which individual women will get breast cancer and which will not.

If relative risks are applied to individuals, it should only be done to assess the annual odds of an event. For example, the risk of getting breast cancer at age 30 is about 1 in 5,900. If a woman has atypical hyperplasia, the relative risk is about 4.4, or 4.4 cases in 5,900 women. To round the numbers off, you might say that the risk is 9 cases in 11,800 women.

If a woman has both atypical hyperplasia and a family history of breast cancer in a first-degree relative (mother or sister), her relative risk of developing breast cancer is 8.9. This means her risk changes from 1 in 5,900 to 1 in 663.

You can also apply the relative risk to the lifetime risk. For example, the lifetime risk of getting breast cancer for a woman who is 50 to 60 years old is 1 in 50. For a woman who is 50, the annual risk is 1 in 590. That means that if a 50-year-old woman has atypical hyperplasia, her risk of getting breast cancer is 4.4 times in 590 women. To round the numbers off, you could say 9 out of 1,180 50-year-old women with atypical hyperplasia will develop breast cancer each year.

The bottom line: When you see the term relative risk used in a study, look more closely to see what the actual events (events is the statistical term for what is being studied, in this instance cancer cases) were in the two groups. Also, remember that large numbers can signal an important finding, but they can also be misleading if the number of events in the two groups is small. They may even be statistically significant, but that doesn't necessarily mean that they are clinically significant, i.e., that they make a difference in how patients should be treated.

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