Just a numbers game? Making sense of health statistics

Presidential candidates use them to persuade voters, drug companies use them to sell their products, and the media spin them in all kinds of ways, but nobody - candidates, reporters, let alone health consumers - understands them. Health statistics fill today's information environment, but even most doctors, who must make daily decisions and recommendations based on numerical data - for instance, to calculate the risks of a certain drug or surgical intervention, or to inform a patient of the possible benefits versus harms of cancer screening - lack the basic statistical literacy they require to make such decisions effectively.

A major new report, "Helping Doctors and Patients Make Sense of Health Statistics," in Psychological Science in the Public Interest, a journal of the Association for Psychological Science, shows that statistical illiteracy is a significant problem having widespread negative impact on healthcare and society. The authors of the report are an international and interdisciplinary team of psychologists — Gerd Gigerenzer and his colleagues Wolfgang Gaissmaier and Elke Kurz-Milcke at the Max Planck Institute for Human Development and Harding Center for Risk Literacy, Berlin, Germany — and physicians — Lisa M. Schwartz and Steven Woloshin at Dartmouth Medical School.

The problem of statistical illiteracy in health has two sides to it, according to Gigerenzer and his team. It is a combined problem caused by the misleading or confusing ways statistics are ordinarily presented in health communication, coupled with a lack of statistical thinking skills among consumers of health information.

The combination can be explosive. A mid-1990s report in Britain showed that new oral contraceptive pills posed a twofold (or 100%) increased risk of blood clots led to a mass panic among women, who opted to stop using this form of birth control as a result. There were an estimated 13,000 more abortions in England and Wales in the year following the report, due to the pill scare. Had the public been informed that the absolute risk with the new pills only increased from 1 in 7,000 women having a blood clot to 2 in 7,000, this panic would not have occurred, and the UK National Health Service would have been saved an estimated $70 million.