Just a numbers game? Making sense of health statistics

It is all too common for health statistics to be presented in terms of relative risks, which are generally large numbers and thus are easily used to capture people's attention. Relative risks ("a 100% increase") are very misleading, however, when important facts like base rates are left out. Absolute risk information (like "1 in 7,000") is much more understandable, and less easy to sensationalize or spin in misleading ways, yet is too often absent from health coverage in the media, pharmaceutical advertising, and even medical journals.

Physicans are not immune. The authors of the report cite several startling studies showing the inability of doctors to accurately interpret data from cancer screenings or other test results, when the data were presented in the typical form of conditional probabilities. For example, when presented with conditional probabilities, a group of experienced physicians was unable to accurately estimate the chances a patient with a positive test result actually had colorectal cancer, given the known sensitivity and false-positive rate of the test used (estimates they gave ranged from a 1% to a 99% chance of having cancer, with most guessing around 50%). Most physicians gave the right answer (a less than 10% chance) when the data were presented in the more transparent form of natural frequencies.

Both an analysis of the problem and a guide for the perplexed, "Helping Doctors and Patients Make Sense of Health Statistics" is an invaluable — and easy-to-understand — guide for correctly making sense of the often wildly misleading statistical data we see every day in the media, on the Internet, and in doctors' offices.

Psychological Science in the Public Interest presents reports on the current state of psychological research on issues of pressing interest to policymakers and society at large. To read the full report on health statistics, please contact Catherine West at 202-293-9300 or .