Science’s Significant Stats Problem

Researchers’ rituals for assessing probability may mislead as much as they enlighten.

By Tom Siegfried Illustration by Sergio Membrillas August 22, 2013
In 2009, researchers working in Thailand made headlines with a small success in a trial of an HIV vaccine. It reduced the rate of infection by 31 percent, the scientists calculated. That may not sound impressive, but in the fight against HIV, it looked like an unprecedented success. The researchers published their results in the influential New England Journal of Medicine, reporting that the data had passed standard statistical tests: If the vaccine had actually been worthless, there was only a 1 in 25 chance that it would have appeared to have the beneficial effect seen in the study.
In medicine, as in most other realms of science, observing low-probability data like that in the HIV study is cause for celebration. Typically, scientists in fields like biology, psychology, and other social sciences rejoice when the chance of a fluke is less than 1 in 20. In some fields, however, such as particle physics, researchers are satisfied only with much lower probabilities, on the order of one chance in 3.5 million. But whatever the threshold, recording low-probability data—data unlikely to be seen if nothing is there to be discovered—is what entitles you to conclude that you’ve made a discovery. Observing low-probability events is at the heart of the scientific method for testing hypotheses.
Scientists use elaborate statistical significance tests to distinguish a fluke from real evidence. But the sad truth is that the standard methods for significance testing are often inadequate to the task. In the case of the HIV vaccine, for instance, further analysis showed the findings not to be as solid as the original statistics suggested. Chances were probably 20 percent or higher that the vaccine was not effective at all.
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