[ View menu ]

Reducing relative risk reduction

Filed in Research News
Subscribe to Decision Science News by Email (one email per week, easy unsubscribe)



Gerd Gigerenzer, Director at the Max Planck Institute for Human Development in Berlin, has a piece in this month’s APA Observer about helping physicians understand screening tests.

Some choice quotes:

Medical doctors tend to think of psychologists as therapists, useful for emotionally disturbed patients, but not for members of their own trade. Research on transparent risk communication is beginning to change that view, however.

As a young researcher, I was struck by a study conducted by David Eddy, now Senior Advisor for Health Policy and Management at Kaiser Permanente. He asked American physicians to estimate the probability that a woman had breast cancer given a positive screening mammogram and provided them with the relevant information: a base rate of 1 percent, a sensitivity of 80 percent, and a false-positive rate of 9.6 percent. Approximately 95 out of 100 physicians wrongly reckoned this probability to be around 75 percent, whereas the correct answer is 7.7 percent (Eddy, 1982). Eddy concluded that many physicians make major errors in statistical thinking that threaten the quality of medical care.

How could doctors not have known the answer? Even if some of the doctors tested were “mathematically challenged,” they should already have known that only about one in 10 women with a positive screening mammogram has cancer. Mammography is one of the most frequently tested medical procedures, with widely published results about its accuracy. But most doctors don’t have the time to read medical journals, and few women know that a positive mammogram is like an activated car alarm — usually a false call. As a result, millions of women who test positive every year are unnecessarily frightened.

Gigerenzer points out that “relative risk reduction” is a much-bandied-about, little-understood way to present information.

Another numerical representation that tends to cloud doctors’ minds is relative risk. We read that mammography screening reduces the risk of dying of breast cancer by 25 percent. Many people believe this to mean that the lives of 250 out of 1,000 women are saved, whereas a group of Swiss gynecologists’ interpretations varied between one in 1,000 and 750 in 1,000! How large is the actual benefit? Randomized trials showed that, out of 1,000 women not screened, four died of breast cancer within about 10 years, whereas among those who were screened, three died. Thus, the absolute risk reduction is one out of 1,000 women, or 0.1 percent, whereas the relative risk reduction is 25 percent. In a representative 2006 survey of 1,000 German citizens, I found that hardly anyone understands what the 25 percent means. Other sources of confusion are single-event probabilities and five-year survival rates.

Photo credit:http://www.flickr.com/photo_zoom.gne?id=362184895&size=m