What if you read that a drug increased the risk of severe injury or death by a huge amount. Would you assume you were more likely to die if you continued taking the drug? Or would you stop taking it?

But statistics about drugs and side effects can be misleading enough that even doctors misunderstand them, ABC News writes about a study in Psychological Science in the Public Interest, which calls many health care professionals “statistical illiterates,” and cites examples of overstated benefits or adverse effects that, in some cases, had devastating results (this is the abstract).

The researchers accuse drugmakers of knowingly using misleading statistics to promote their products because a “big number” can lead to a big headline and lots of sales, ABC News reports, adding that a well-meaning institution might also seek a big number to warn of possible dangerous side effects of a drug, even if that number implies a much greater risk than actually exists.

“Many doctors, patients, journalists, and politicians alike do not understand what health statistics mean,” according to the study, authored by two medical professors at Dartmouth Medical School, Steven Woloshin and Lisa M. Schwartz, and their collaborators, psychologists Gerd Gigerenzer, Wolfgang Gaissmaier and Elke Kurz-Milcke of the Max Planck Institute for Human Development in Berlin.

One case - In 1995, the UK Committee on Safety of Medicines warned that “third-generation oral contraceptive pills increased the risk of potentially life-threatening blood clots in the legs or lungs twofold - that is, by 100 percent,” the report states. But the study says this was misleading and caused a scare that was blamed for 13,000 subsequent abortions, many involving teen pregancies.

“This information was passed on in ‘Dear Doctor’ letters to 190,000 general practitioners, pharmacists, and directors of public health and was presented in an emergency announcement to the media,” the report continues. “The news caused great anxiety, and distressed women stopped taking the pill, which led to unwanted pregnancies and abortions.”

The studies on which the warning was based had shown that, of every 7,000 women who took the earlier, second-generation oral contraceptive pills, about one had a thrombosis; this number increased to two among women who took third-generation pills, ABC News writes. The absolute risk increase was only one in 7,000, whereas the relative increase (among women who developed blood clots) was indeed 100 percent.”

That was an increase from one to two, but out of 7,000. ABC News writes that the omission of key info is common to many claims about drugs and medical procedures, regardless of the source of the claims, according to the study. It’s routine in drug ads and common in announcements from many institutions that have found a dangerous side effect and want to attract consumer attention.

There are several other ways that statistics can be very misleading, like overstating survival rates, or implying that in all cases early screening can lead to early detection and more successful treatment, according to the study.

In a campaign ad, Rudy Giuliani said: “I had prostate cancer, 5, 6 years ago. My chance of surviving prostate cancer - and thank God, I was cured of it - in the United States? Eighty-two percent. My chance of surviving prostate cancer in England? Only 44 percent under socialized medicine.” ABC News notes that the reseachers contend that was wrong: “Giuliani’s numbers are meaningless for making comparisons across groups of people that differ dramatically in how the diagnosis is made.”

You can read the rest here…