2/n The study is preprinted on medrxiv here. It is a single-author study on a survey done in India during COVID-19 https://www.medrxiv.org/content/10.1101/2021.02.12.21249710v1 … This will be a short thread, because...wow. Issues
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3/n Some background here - traditionally, in epidemiology, to work out whether something is protective against disease, you need to know two basic things: 1. Likelihood of disease if exposed 2. Likelihood of disease if not exposed
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4/n Essentially, to know whether glasses make you less likely to catch COVID-19, we'd need to know whether people with glasses caught the disease less than people without glasses Simple, right?pic.twitter.com/1GxB0YwOuD
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5/n Ok, back to the study What did the author do? Well, the asked 304 people with COVID-19 whether they wore glasses most/all of the time. 58 (19%) said yespic.twitter.com/ct1P60oIgK
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6/n Then, the author took the proportion of Indian adults estimated to wear glasses from a paper in 2019, to compare this sample topic.twitter.com/tYRVgFZElZ
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7/n So far so problematic. You can't just compare to an out-of-study population like that, it makes no sense at all. If nothing else, the comparison group is for the whole of India, while this survey was done on a tiny subsample in one hospital
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8/n But then, we get to these calculations, which are described as "the risk of [catching] COVID-19" in glasses wearers vs non-wearers But...that's just incorrectpic.twitter.com/bkyRhQkwT3
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9/n What the author has done here is compare the rate of glasses wearing in COVID-19 patients to the rate of glasses wearing in the general populationpic.twitter.com/wwC3Mw2qHQ
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10/n What this calculation actually gives you is the likelihood of wearing glasses in COVID-19 vs non-COVID-19 people In other words, what we've got here is the relative risk of glasses-wearing behaviour in COVID-19 patients compared to everyone else
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11/n So not only is the study a tiny cross-sectional survey with no appropriate comparator, it's also not calculating what the headlines (and conclusions) say it is at all
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12/n What the headlines should say is that COVID-19 patients are 2-3 times less likely to wear glasses than the general population, based on a small, biased survey But I doubt that will get as much attention, because it's a bit meaningless
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13/n Here is the conclusion of the study. This is not correct based on the methodology as described:pic.twitter.com/SRaHx0zzZ8
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14/n Oh and in case anyone was wondering, according to Altmetric the study has been in 51 news articles so farpic.twitter.com/dSFahe908r
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