A very, very impressive cluster randomized trial. Very hard to do! See thread for findings.https://twitter.com/Jabaluck/status/1433036923610742789 …
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It's usually hard to show any effect with this type of trial due to statistical power limitations. But they find some anyway, despite also having other significant factors pulling against finding an effect besides just the thorny, inherent difficulty in cluster RCTs!
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They find effects despite neither reaching universal mask coverage (they can't force, only nudge and they reach 42%) nor having a control with no masking (since would be unethical) *and* being conducted in a much lower-risk setting—rural Bangladesh, few crowded sealed buildings.
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They find that, even with a design necessarily biased against finding effects due to statistical power limits and ethical barriers to having a no-mask control group, protective effect of masks is stronger for surgical masks than cloth masks, and significant for the elderly.
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Honestly, when the authors told me, I was skeptical it could be done and the design limits would overpower— unavoidable: cluster randomization has limited power especially for overdispersed phenomena and you can't confine people to one village or order controls not to wear masks.
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Do note that when you have a study design that really works against being able to detect an effect, the key interpretation is relative—within study. Don't read anyone who doesn't understand statistical power, cluster designs, overdispersion and the ethical limits of controls.
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Parts that surprised me: surgical masks retain filtration through many, many washing. Part that did not surprise me, almost nothing the (pre-polled) policy makers thought would work to nudge mask use higher worked. Policy folk and actual human behavior don't talk much, sadly.
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Part that doesn't surprise me: people who will conflate what a randomized cluster trial measuring an ecological effect in a real life setting with a randomized *clinical* trial (where we control everything and don't deal with overdispersion and only measure individual benefits).
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Replying to @zeynep
This study is absolutely horrific from an ethical and historical standpoint. It should never be published.
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Replying to @qdurandmoreau
Why? I was super skeptical a study like this could be done, but they did it by doing it ethically and (correctly) sacrificing statistical power. The statistical controls are really weak, to be honest, exactly because it is ethical.
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Also look at how wrong the policy makers were on what would work to nudge mask use. Gazillion dollars are being spent on what policy makers imagine people are (and they are usually wrong) and on that aspect, they have proper controls and show what works. Super valuable.
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Replying to @zeynep
We really do not need this to support masking and proper ventilation controls. We already have way more than we need about airborne transmission of Covid.
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Replying to @qdurandmoreau
Agree this wasn't sine qua non for supporting masking or ventilation. But look at the bulk of the study: on how to nudge and how wrong policymakers are on this. That is *super* valuable, and as you see, because of (correct ethical) limits to controls, rest is limited in power.
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