With Covid studies, the quality of the evidence matters | David Spiegelhalter and Anthony Masters

Putting many low-quality studies together cannot provide reliable answers about masks and ivermectin

In The Adventure of the Copper Beeches, Sherlock Holmes says: “Data! Data! Data! I can’t make bricks without clay.” Recent claims of massive benefits from wearing masks and using ivermectin against Covid-19 depended on mainly low-quality clay.

Meta-analysis is a technique for pooling the results from many studies, but it cannot make silk purses out of sows’ ears. A recent British Medical Journal review looked at six, fairly porcine, studies concerning mask-wearing and estimated an impressive 53% reduction in risk. But the single randomised controlled trial estimated the smallest effect: a reduction of about 18% (-23% to 46%) in Sars-CoV-2 infections. The “heaviest” studies, an analysis of US states and a survey of about 8,000 Chinese adults in early 2020, observed rather than experimented and its editorial highlights the risks of confounding variables influencing both wearing masks and infections and the impossibility of disentangling the effects of measures fluctuating simultaneously. Indeed, this review found an identical 53% reduction from handwashing.

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