The likelihood of a given unwanted event, such as an accident, the development of a disease, or the occurrence of a side effect. In reports about medical research, the way risk is presented can affect how worrisome it seems.

In medical studies, risk is often described in terms of “relative risk” and “absolute risk.” Say a study has found that a drug raises the likelihood of an adverse effect. The relative risk is a number that compares the chance that people taking the drug will experience the adverse effect to the chance that people not taking the drug will experience it. If people who take the drug and people who don’t take it have an equal chance of experiencing the effect, then the relative risk is said to be 1.0. If the effect occurs in twice as many people who take the drug as it does in people who don’t take the drug, then the relative risk is said to be 2.0. Another way to express the relative risk in the latter instance is to say that the drug “doubles” the risk of the adverse effect.

The problem with relative risk is that it can make the situation sound more alarming than it really is. For example, you might hear that a drug you’re considering taking triples the risk of cancer. If you knew how many cases of cancer this actually amounts to, it might not sound quite so alarming.

That’s why absolute risk is often a more useful number. Absolute risk is the number of people who experience the adverse effect. It’s usually expressed as a ratio, such as “3 in 1,000” or “50 out of every 100,000.” Knowing the absolute risk helps to put the relative risk into perspective. In the example of the drug that triples the risk of cancer, if you knew that only 5 out of every 100,000 people who didn’t take the drug got cancer in the first place, you’d be able to see that the drug doesn’t cause that many extra cases of cancer. It raises the absolute risk to 15 out of every 100,000 people. If you look at the risk this way, you might decide that it’s not big enough to stop you from taking the drug.

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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