The word “risk” often connotes danger: The risk of getting cancer. But in medicine, risk is a ratio that’s used to show the chance that something will happen. There are two main kinds of risk to find and report when covering a medical study—absolute risk and relative risk.
Deeper dive
Relative risk is often the headline generating number that’s pulled out of a medical study: “Study shows Experimental Drug Halves Heart Attack Risk.”
Without also reporting absolute risk, however, relative risk can exaggerate either the chance of getting sick or the chance that a drug or treatment may help. [See “Tanning beds: What do the numbers really mean?” for an example.]
Absolute risk is the overall chance that something will happen. It’s also sometimes referred to as the starting risk. In randomized-controlled trials, absolute or starting risk is usually found by looking at what happens to the group assigned to take the placebo.
For example, in a study testing a new drug against a placebo to prevent heart attacks, if 40 people out of 1,000 who are taking a placebo die of heart attacks during the study the absolute risk of dying of a heart attack during the study was 40 out of 1,000 or 40 ÷ 1,000 = .04 or 4 percent.
Now let’s say that in the same study, the risk of dying of a heart attack among people taking an experimental drug was 20 out of 1,000 or 20 ÷ 1,000 = .02 or 2 percent.
Relative risk is calculated by comparing the risk of a heart attack in the group that received the experimental treatment to the risk seen in the placebo group. In this example, it would be calculated like this .02 ÷ .04 = .5 or 50 percent.
The study would say that the relative risk of a heart attack among people who were taking the study drug was .50, or 50 percent.
In reporting the study, a writer might say: Overall, 4 percent of people in the placebo group suffered heart attacks during the course of the study compared to 2 percent in the group on the experimental drug. The new drug reduced a person’s chances of having a heart attack by 50 percent.
This kind of graph is helpful for readers because it shows two things: First, that the starting risk of having a heart attack during the study was pretty low; and second, the relative effect of the new drug.
Obviously, this is very simplified example. The numbers presented in actual scientific papers are generally more complex, and they may be expressed in different ways. If you’re not easily able to find the absolute risk behind a relative risk, don’t be afraid to ask the researcher. They’re usually happy to explain.