This is a guest post by Frederik Joelving, a staff writer at Reuters Health.
Absolute risk is one of the biggest buzzwords in health reporting today, and for good reasons.
It’s frightening to hear that hormone replacement therapy doubles your risk of suffering a blood clot in your lungs, for instance (the relative risk). But knowing that in fact it causes fewer than one such event per 1,000 women per year puts the risk in perspective (the absolute risk).
As a Cochrane review pointed out this week, relative risks are very persuasive, but they don’t always serve your best interests when making health decisions.
So why shouldn’t we as reporters just stick to absolute risks?
The problem is that unless you’re dealing with a large randomized controlled trial, absolute risks can be misleading, too. They carry hidden baggage, such as age, overall health, ethnicity and so on — all things that on their own could influence your risk of getting sick.
Here’s an example, which inspired me to write this post: When HealthNewsReview.org reviewed a recent story I wrote on a study of bone drugs and colon cancer, they pointed out its lack of absolute numbers:
The story should have said that 138 women in the non-cancer group took bisphosphonates and 97 in the cancer group took them, meaning that 41 women appear to have benefited from taking the drugs.
Instead of giving those figures, I chose to report the relative risk reduction (59 percent) and the lifetime risk in the general population (5 percent).
Why? Because even in a case-control study like the one I was writing about, there are bound to be important differences between the two patient groups — such as their general health, medication use and diet. Those differences make the absolute numbers hard to interpret at best, and misleading at worst.
In this and similar cases, I think you’re better off knowing the baseline risk in the general population. That gives you a sense of whether you should worry about colon cancer in the first place.
Once you know that, the relative risk starts to make sense. That’s because researchers usually do us the favor of trying to weed out the influence of extraneous factors in a so-called multivariate analysis.
What you end up with is the best estimate of the actual effect of bisphosphonates on colon cancer, assuming the link is causal (which of course we can’t in this case, because the study is observational).
Another example: About a month ago, researchers reported that 17 percent of people with mild hearing loss had dementia, compared to 4 percent of people with normal hearing.
Those are absolute numbers, but they won’t tell you much about your own risk of dementia, even if you happen to be hard of hearing. The main reason is that hearing loss tends to go hand-in-hand with getting old, as does dementia risk, and the numbers don’t take that into account. That’s why we didn’t include them.
The relative increase in risk — a doubling in this case — doesn’t tell you much about your own risk either, not least because it’s impossible to account for all potentially important variables.
But it tells you something real about the strength of the connection between the two phenomena, dementia and hearing loss. And more importantly, it doesn’t give you a false sense of knowing your own risk.
Absolute risk is an important measure, but we shouldn’t use it indiscriminately.
Publisher’s note: I welcome this kind of contribution from journalists whom we review. I hope for a broader, more open dialogue and exchange of ideas in the future. For years now we have offered a brief primer on absolute versus relative risks on our website.