The CDC’s facemask failure

Jacob Sullum:

new study published by the Centers for Disease Control and Prevention (CDC) supposedly shows that wearing a face mask in public places dramatically reduces your risk of catching COVID-19. The CDC summed up the results in a widely shared graphic that says wearing a cloth mask “lowered the odds of testing positive” by 56 percent, while the risk reduction was 66 percent for surgical masks and 83 percent for N95 or KN95 respirators.

If you read the tiny footnotes, you will see that the result for cloth masks was not statistically significant. So even on its face, this study, which was published in the CDC’s Morbidity and Mortality Weekly Report on Friday, did not validate the protective effect of the most commonly used face coverings—a striking fact that the authors do not mention until the end of the sixth paragraph. And once you delve into the details of the study, it becomes clear that the results for surgical masks and N95s, while statistically significant, do not actually demonstrate a cause-and-effect relationship, contrary to the way the CDC is framing them.

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