If you were told a drug you are taking has been found to increase the risks of death by 100 percent, would you stop taking it? Of course. But does that statement really mean you are much more likely to die if you keep taking the drug? Absolutely not.
Statistics like that, touting drugs and their possible side effects, can be so misleading that even doctors frequently misunderstand them, according to a major international study that brands many health care professionals as "statistical illiterates."
The report, published in the current issue of Psychological Science in the Public Interest, cites numerous instances of grossly overstated benefits, or adverse effects, of drugs and medical procedures, which, in some cases, has had devastating results.
The researchers accuse drug companies of knowingly using misleading statistics to promote their products because a "big number" can lead to a big headline and lots of sales. And 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.
Statistical skullduggery strikes at just about every level, from professional medical journals to health care workers to medical writers who convey the bad numbers that frequently come from news releases promulgated by some of the leading research institutions in the world, the report says.
"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 of many cases they cite had tragic consequences. In 1995, the United Kingdom Committee on Safety of Medicines issued a warning 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. It was a big number, and it got a lot of attention, but it was very misleading.
"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 scare was blamed for 13,000 abortions the following year, many involving teen pregnancies.
But who can argue with 100 percent? The report adds:
"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. 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's right. An increase from one to two, but that's out of 7,000.
That omission of key information, namely the numbers, is common to many -- perhaps most -- claims about drugs and medical interventions, regardless of the source of the claims, according to the study. It's routine in drug advertisements. It's common in announcements from many institutions that have found a dangerous side effect and want to be sure they get your attention.
But it's wrong.
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, the researchers contend.
In his unsuccessful attempt to win his party's nomination for president, Rudy Giuliani said in a campaign advertisement: "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."
That is flat out wrong, according to the researchers. "Giuliani's numbers are meaningless for making comparisons across groups of people that differ dramatically in how the diagnosis is made," the report states.
Giuliani's claim that men are nearly twice as likely to survive in the United States as in England is based on a five-year survival rate after detection. In a 2000 study, 49 British men per 100,000 were diagnosed with prostate cancer, of which 28 died within five years, about 44 percent. But screening for prostate cancer is different in England than in the United States. This country relies heavily on a prostate-specific antigens (PSA) test, which can sometimes detect cancer earlier. But the test is not widely used in England. Thus, the detection, and the five-year survival rate, spans a different time in the lives of patients in the two countries.
To illustrate the point, the report turns to a hypothetical situation:
"Imagine a group of prostate cancer patients currently diagnosed at age 67, all of whom die at age 70. Each survived only three years, so the five-year survival of this group is 0 percent. Now imagine that the same group is diagnosed with prostate cancer by PSA tests earlier, at age 60, but they all still die at age 70."
If the patients in the second group lived to the age of 65, their five-year survival rate would be 100 percent, although they all died by age 70.
"Even though the survival rate has changed dramatically, nothing has changed about the time of death," the report notes.
"Are American men half as likely to die from prostate cancer as British men are?" the study continues. "The answer is no; the risk is about the same: About 26 prostate cancer deaths per 100,000 American men versus 27 per 100,000 in Britain."
Not to worry, the doctor always knows best, right? Not necessarily, according to the report.
In several studies, physicians were asked if they really understood what the widely known 25 percent risk reduction by mammography screening really means. If 1,000 women are tested, how many fewer will die of breast cancer?
The answers were all over the map, ranging from one out of 1,000 to 750 out of 1,000. Fortunately, most gave the correct answer, the researchers say, which is one. One more survivor will be added to the list, and that's 25 percent of the total number of survivors. But even after the answer was revealed, one physician insisted it should be 250.