Out of How Many? The Question That Makes Health Headlines Make Sense

By Mark J Kaylor

You’ve seen the headline. A food, a supplement or a habit “doubles your risk” of something awful. Or a new product “cuts your risk in half.” Your stomach drops, or your hopes rise, and either way you’re about to make a decision.

One small question helps a lot: out of how many?

Researchers can report the same result in two ways. Relative risk compares two groups to each other.

Absolute risk tells you what actually happens to people.

Say 1 in 10,000 people develop a certain condition. In a study, people who eat a certain food develop it at 2 in 10,000. That’s a 100% increase in relative risk, which makes a dramatic headline. In absolute terms, the risk went up by 1 in 10,000. Both numbers are accurate, but only one helps you decide what to do.

It works in the other direction too. “Cuts your risk by 50%” sounds like a sure thing, until you learn the risk went from 2 in 1,000 to 1 in 1,000.

The relative number is real, but it’s only half the picture. The trouble starts when a headline, an ad or an influencer gives you only that half and doesn’t say which kind of risk they’re quoting. Sometimes the absolute number is right there in the study and just didn’t make the headline. Sometimes you have to dig for it.

So when a number makes you want to act, pause and ask: out of how many, and compared to what? That one question puts the decision back in your hands.