By H. Gilbert Welch

Numerous studies have shown that the general public has exaggerated perceptions of the health risks they face — as well as exaggerated expectations of the benefit of medical care.

Is it because they’re stupid? No. Instead, the problem relates to how various sources of health information — researchers, doctors, reporters, web designers, advertisers, etc. — frequently frame their messages: using relative change.

“Forty percent higher” and “50 percent lower” are statements of relative change. While they are easy to understand, they are also incomplete. Relative change can dramatically exaggerate the underlying effect. It’s a great way to scare people.

For example, research earlier this year found that women with migraines had a 40 percent higher chance of developing multiple sclerosis. That sounds scary.

But the researchers were careful to add some important context: Multiple sclerosis is a rare disease. In fact, for women with migraines, the chance of developing multiple sclerosis over 15 years was considerably less than 1 in 100 — only 0.47 percent. To be sure, that is about 40% higher than the analogous risk for women without migraines — 0.32 percent — but it’s a lot less scary. More importantly, it’s a much more complete piece of information.

What makes it more complete is the context of two additional numbers: the risk of developing multiple sclerosis in women with and without migraines. Epidemiologists call these “absolute risks.” You and I might call them the real numbers.

Relative change also exaggerate effects in the other direction. It’s a great way to make people believe there has been a real medical breakthrough.

A few years ago a study of a cholesterol-lowering statin drug was hailed for big reductions in heart attacks in people with so-called healthy cholesterol levels. The drug led to about a 50 percent reduction in the risk of heart attack. That sounds like a breakthrough.

But the absolute risks — the real numbers — are sure to look a little different. Why? Because in people with healthy cholesterol levels, heart attacks are rare. To get that context, get the two additional numbers: the risk of heart attack in people taking and not taking the drug.

For people taking the drug, the chance of having a heart attack over five years was less than 1 percent. To be sure, that is about 50 percent lower than the analogous risk for those not taking the drug — less than 2 percent — but it sounds a lot less like a breakthrough.

These absolute risks suggest that 100 apparently healthy people have to take the medication for five years for one to avoid a heart attack. And it’s not even clear from the research — or the federal registry of clinical trials — what kind of heart attack: the kind that patients experience (the bad kind) or the kind that is diagnosed by detecting less than a billionth of gram of a protein in the blood (the not-so-important kind). Add in all the hassle factors of being on another drug (filling scripts, blood tests, insurance forms) and the legitimate concerns about side effects, the use of relative change might now strike you as more than a little misleading.

Whatever the finding — harm or benefit — relative change exaggerates it.

Upon learning this, one of my students likened relative change to funhouse mirrors. If you are thin, there is a mirror that can make you look too thin; if you are heavy, there is mirror that can make you look too heavy.

In the case of relative change, it all happens in the same mirror. It provides a potent combo to promote medical care: exaggerated perceptions of risk and exaggerated perceptions of benefit. Can you imagine a more powerful marketing strategy?

Relative change is not the only culprit in misleading health information, but it is an important one. The good news is that more and more researchers, reporters and editors are on to this game. The bad news is that there is an awful lot of information to police and sometimes it can be hard to even find the real numbers.

That’s where a skeptical, numerate public comes in — one that knows to ask for the real numbers. And, if they can’t be found, one that knows to move on.

H. Gilbert Welch is a professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice. He is the coauthor of Overdiagnosed: Making People Sick in the Pursuit of Health. This post originally appeared on The Huffington Post.