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Lies, Damned Lies, and Statistics

Another Look at Study Reporting Universal Mask Policy Pulls Down Positivity Rate Among Health Care Workers

While masks likely do have substantial benefit, this study doesn’t show it.

A new study in the Journal of the American Medical Association reports that a universal mask policy at a Massachusetts health system led to a “significantly lower rate of [COVID] positivity” test rates among health care workers.

While masks likely do have substantial benefit, this study doesn’t show it. It’s important to maintain standards of high-quality science, even if the conclusion is something that seems obviously true.

The study merely looks at COVID tests given to the health care workers, and plots out the positivity rate — that is, the percent of COVID tests that return a positive result — from March 6 to April 29. It finds that the positivity rate started at zero (at the outset of COVID), escalated rapidly in March, and then when masking went into effect at the health system, the rate thereafter “decreased linearly from 14.65% to 11.46%.”

That’s it. There is no actual comparison group, except for one sentence mentioning that “the case number continued to increase in Massachusetts throughout the study period.” But that was obviously true of the health system itself as well (or else the positivity rate would have dropped to zero again).

Worse, the citation for that claim is a state report that plots the positivity rate for the entire state, which would have been a much more useful comparison. It turns out that between April 11 and April 29, the statewide positivity rate dropped from 29-33% to 17-19%. That’s a more substantial drop than occurred at the health system in question. Given other statewide interventions at the same time (such as a stay-home order), it’s impossible to attribute a modest decline at one health system to the mask policy there.