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GidMK's profile
Health Nerd
Health Nerd
Health Nerd
Verified account
@GidMK

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Health NerdVerified account

@GidMK

Epidemiologist. Writer (Guardian, Observer etc). "Well known research trouble-maker". PhDing at @UoW Host of @senscipod Email gidmk.healthnerd@gmail.com he/him

Sydney, New South Wales
theguardian.com/profile/gideon…
Joined November 2015

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    1. Health Nerd‏Verified account @GidMK 17 Sep 2020

      It's been coming up a lot lately, so I thought I'd do a bit of a thread on CONVENIENCE SAMPLES and why they aren't great for assessing POPULATION PREVALENCE of a disease In other words - how many people have had COVID-19? 1/n

      4 replies 40 retweets 109 likes
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    2. Health Nerd‏Verified account @GidMK 17 Sep 2020

      2/n So, the basic idea here is simple. We want to know about people who have (or in this case, have had) a disease How do we find that out?pic.twitter.com/e3whaw7Xjz

      1 reply 0 retweets 9 likes
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    3. Health Nerd‏Verified account @GidMK 17 Sep 2020

      3/n The traditional method is to do a large, randomly-sampled study involving dialing up 10,000s of people across a population and surveying them + doing lots of blood tests But this is EXPENSIVE

      2 replies 0 retweets 12 likes
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    4. Health Nerd‏Verified account @GidMK 17 Sep 2020

      4/n Running a proper statistically representative process, getting all the people to answer their phones and give you bloods...even if the cost per person is low, multiply that by 10-100,000 and the cost can be prohibitive

      1 reply 0 retweets 10 likes
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    5. Health Nerd‏Verified account @GidMK 17 Sep 2020

      5/n Which brings us to the idea of a CONVENIENCE SAMPLE Why is it called a convenience sample (hint: answer is in the name)

      1 reply 0 retweets 9 likes
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    6. Health Nerd‏Verified account @GidMK 17 Sep 2020

      6/n Yes, convenience samples are just that - convenient Usually, they are groups of people that you are ALREADY TESTING for some reason that you can either add another test on to or survey

      1 reply 0 retweets 12 likes
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    7. Health Nerd‏Verified account @GidMK 17 Sep 2020

      7/n I have used this method in the past to look at the burden of diabetes in-hospital and GP clinics - we looked at people who were already getting blood tests, and added one extra test for diabetes (and science!) https://www.sciencedirect.com/science/article/abs/pii/S0168822718318862 …pic.twitter.com/HoYla4gXSD

      1 reply 0 retweets 12 likes
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      Health Nerd‏Verified account @GidMK 17 Sep 2020

      8/n But there's an issue here We have selected these people very specifically. They are not a random, representative sample - they were people ALREADY GETTING blood tests which means they are probably different in LOTS OF WAYS to the general population

      1:03 AM - 17 Sep 2020
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      1 reply 1 retweet 18 likes
        1. New conversation
        2. Health Nerd‏Verified account @GidMK 17 Sep 2020

          9/n So in our lovely study of a convenience sample of diabetes tests, we can't say anything about how much diabetes there is in the community (population prevalence)! All we can talk about is diabetes IN THE PATIENTS TESTED

          1 reply 0 retweets 13 likes
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        3. Health Nerd‏Verified account @GidMK 17 Sep 2020

          10/ "But God", you ask, with a common autocorrect mistake, "what does this have to do with COVID-19?" Well, reader, this is where we get to antibody testingpic.twitter.com/OdmE1jOyKX

          1 reply 0 retweets 17 likes
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        4. Health Nerd‏Verified account @GidMK 17 Sep 2020

          11/n You see, when you get sick with a new disease, your body produces antibodies* We can then test for these antibodies to see if you've had the disease before* *oversimplified, plz don't murder me immunologists

          1 reply 0 retweets 16 likes
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        5. Health Nerd‏Verified account @GidMK 17 Sep 2020

          12/n If you run an antibody test on a large group of people, it's called a serosurvey (because antibody tests are also known as serology in sciency terms)

          1 reply 0 retweets 13 likes
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        6. Health Nerd‏Verified account @GidMK 17 Sep 2020

          13/n Now, a lot of places (countries, states, colleges) have run serosurveys and had a grand old time of it. This is why you keep seeing those news articles saying that x% of people in a place have had COVID-19 alreadypic.twitter.com/PJbt4tWiYb

          1 reply 1 retweet 11 likes
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        7. Health Nerd‏Verified account @GidMK 17 Sep 2020

          14/n The problem is, some of these serosurveys used CONVENIENCE SAMPLES Just like we discussed earlier, that makes them a bit problematic

          1 reply 1 retweet 14 likes
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        8. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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 …

          1 reply 0 retweets 13 likes
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        9. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 2 retweets 15 likes
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        10. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          2 replies 2 retweets 17 likes
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        11. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 1 retweet 20 likes
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        12. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 1 retweet 13 likes
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        13. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 0 retweets 12 likes
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        14. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 2 retweets 17 likes
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        15. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 2 retweets 21 likes
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        16. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 1 retweet 15 likes
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        17. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          1 reply 0 retweets 17 likes
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        18. Health Nerd‏Verified account @GidMK 17 Sep 2020

          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

          4 replies 0 retweets 19 likes
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        19. End of conversation

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