14/n The problem is, some of these serosurveys used CONVENIENCE SAMPLES Just like we discussed earlier, that makes them a bit problematic
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15/n My co-authors and I, in our systematic review of age-stratified IFRs for COVID-19, looked into just how problematic The answer: a whole lothttps://www.medrxiv.org/content/10.1101/2020.07.23.20160895v4 …
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16/n For example, one study in Tokyo that used a CONVENIENCE SAMPLE found that 3.8% of people had had COVID-19 in the sample tested But a proper randomized sample found just 0.1% - 38 times lower!pic.twitter.com/3qTdzCXVXX
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17/n In England, a CONVENIENCE SAMPLE of blood donors implied that 1 in 12 people had had COVID-19, but a large representative sample found it was just 1 in 20pic.twitter.com/XyTVSL6LDs
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18/n The problem is, these CONVENIENCE SAMPLES are systematically biased. They are of people who are different to the general population in ways that can be very difficult to measure and/or understand
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19/n Blood donors, for example, are young and healthy by design. But the people who have been (generously) giving blood during the pandemic might also be...well, a bit odd
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20/n They're going to great personal lengths to sacrifice for the rest of us ungrateful buggers, which might indicate that they're more likely to socialize, more likely to mingle, and thus more likely to get infected We JUST DON'T KNOW
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21/n And this is the problem with convenience samples, generally We cannot use them to estimate population prevalence (how many people have had COVID-19), because they aren't representative of society as a whole
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22/n So if you see a headline that says "x% of people infected with COVID-19!" take a leaf out of my mentor's book and ask: "WHAT'S THE DENOMINATOR?" It's a vitally important question
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23/n THIS DOESN'T MEAN THAT CONVENIENCE SAMPLES ARE USELESS I use them in my research. They are brilliant for quick, cheap tracking of rates of infection IN SELECT GROUPS They also provide a brilliant window into change OVER TIME
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24/n For example, if you sample blood donors every week for a year, you've got an amazing insight into the changing nature of the pandemic THIS IS MASSIVELY IMPORTANT AND VERY CHEAP
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25/n You just can't use those results to tell how many people in the rest of society have gotten COVID-19 But that doesn't mean the results aren't helpful at all
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