Tag Archives: relative risk

Understanding risk, and why 83% is really 10.5%

Andrew Van Dam

About Andrew Van Dam

Andrew Van Dam of The Wall Street Journal previously worked at the AHCJ offices while earning his master’s degree at the Missouri School of Journalism, and he has blogged for Covering Health ever since.

Physician and blogger Alex Lickerman used a discussion he had regarding hormone replacement therapy and breast cancer as an opportunity to explain absolute versus relative risk. It’s a an issue we’ve tackled in Covering Health before (Thanks, Ivan), but the difference between an 83 percent jump in relative risk and a 10.5 percent increase in absolute risk is so fundamental that we’re happy to spotlight Lickerman’s particularly newsworthy example.

In case you don’t have time for Lickerman’s entire tale, here’s one of his smaller examples. It’s a blunter version of the same overall point.

Studies show in patients who’ve had a heart attack that taking one aspirin a day reduces their relative risk of having a heart attack over nearly a 10-year period by almost 50%. In patients over the age of 80, for example, whose absolute risk of having a heart attack can be as high as 12% in just the first six months following their first heart attack, this amounts to a recalculated absolute risk of 6%. Arguably still significant, but not nearly as much as the 50% relative risk reduction commonly bandied about in medical circles.

Related

For more on relative and absolute risk, as well as other statistical concepts that are important in health journalism, be sure to download AHCJ’s latest slim guide, “Covering Medical Research.”

Covering Medical ResearchThe guide helps journalists analyze and write about health and medical research studies. It offers advice on recognizing and reporting the problems, limitations and backstory of a study, as well as publication biases in medical journals. It includes 10 questions you should answer to produce a meaningful and appropriately skeptical report. This guide, supported by the Robert Wood Johnson Foundation, will be a road map to help you do a better job of explaining research results for your audience.

Tanning beds: What do the numbers really mean?

Pia Christensen

About Pia Christensen

Pia Christensen (@AHCJ_Pia) is the managing editor/online services for AHCJ. She manages the content and development of healthjournalism.org, coordinates social media efforts of AHCJ and assists with the editing and production of association guides, programs and newsletters.

This is a guest post from Ivan Oransky, M.D., editor of Reuters Health and AHCJ’s treasurer, written at my invitation.

May has been declared “Melanoma Awareness Month” or “Skin Cancer Awareness Month” – depending on which group is pitching you – and reporters are doubtlessly receiving press releases and announcements from a number of groups, including the Melanoma Research Foundation, the Skin Cancer Foundation, hospitals, doctors and other organizations.

Those press releases often point to the World Health Organization, which reports that “use of sunbeds before the age of 35 is associated with a 75% increase in the risk of melanoma” – a statistic often repeated in news stories about tanning beds.

tanning-bedPhoto by Whatsername? via Flickr

But what does that really mean? Is it 75 percent greater than an already-high risk, or a tiny one? If you read the FDA’s “Indoor Tanning: The Risks of Ultraviolet Rays,” or a number of other documents from the WHO and skin cancer foundations, you won’t find your actual risk.

That led AHCJ member Hiran Ratnayake to look into the issue in March for The (Wilmington, Del.) News Journal, after Delaware passed laws limiting teens’ access to tanning salons. The 75 percent figure is based on a review of a number of studies, Ratnayake learned. The strongest such study was one that followed more than 100,000 women over eight years.

But as Ratnayake noted, that study “found that less than three-tenths of 1 percent who tanned frequently developed melanoma while less than two-tenths of 1 percent who didn’t tan developed melanoma.” That’s actually about a 55 percent increase, but when the study was pooled with others, the average was a 75 percent increase. In other words, even if the risk of melanoma was 75 percent greater than two-tenths of one percent, rather than 55 percent greater, it would still be far below one percent.

For some perspective on those numbers, Ratnayake interviewed Lisa Schwartz, M.D.,M.S., whose work on statistical problems in studies and media reports is probably familiar to many AHCJ members. “Melanoma is pretty rare and almost all the time, the way to make it look scarier is to present the relative change, the 75 percent increase, rather than to point out that it is still really rare,” Schwartz, a general internist at Veterans Affairs Medical Center in White River Junction, Vt., told him.

In a nutshell, the difference between skin doctors’ point of view and Schwartz’s is the difference between relative risk and absolute risk. Absolute risk just tells you the chance of something happening, while relative risk tells you how that risk compares to another risk, as a ratio. If a risk doubles, for example, that’s a relative risk of 2, or 200 percent. If it halves, it’s .5, or 50 percent. Generally, when you’re dealing with small absolute risks, as we are with melanoma, the relative risk differences will seem much greater than the absolute risk differences. You can see how if someone is lobbying to ban something – or, in the case of a new drug, trying to show a dramatic effect –  they would probably want to use the relative risk.

This is not an argument for or against tanning beds. It’s an argument for clear explanations of the data behind policy decisions. For some people, the cosmetic benefits of tanning beds – and the benefit of vitamin D, for which there are, of course, other sources – might be worth a tiny increase in the risk of melanoma. For others, any increased risk of skin cancer is unacceptable. (And of course, for the tanning industry, the benefits can be measured in other ways – dollars.) But if reporters leave things at “a 75 percent increase,” you’re not giving your readers the most important information they need to judge for themselves.

So when you read a study that says something doubles the risk of some terrible disease, ask: Doubles from what to what?

Related

These numbers also might come up in reporting about the health reform bill as it does in “Indoor Tanning Getting Moment in the Sun” (March 26, 2010). From the story:

Over the past decade, indoor tanning has increasingly been likened to other maligned habits, cigarette smoking in particular.

And with the passage of the new health care bill, government officials are prepared to take that comparison one step further. A 10 percent tax could be levied on indoor tanning as early as July, in an effort to offset some of the health care bill’s multi-billion-dollar budget.

AHCJ resources on writing about medical studies:

In addition, look for a slim guide about covering medical studies that AHCJ will publish this summer.