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GidMK's profile
Health Nerd
Health Nerd
Health Nerd
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@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 6 Jun 2018

      There's been a lot of controversy about drug sniffing dogs. Here's why they are totally useless as an intervention (thread)

      5 replies 7 retweets 19 likes
      Show this thread
    2. Health Nerd‏Verified account @GidMK 6 Jun 2018

      We want to know how accurate a test sniffer dogs are. To put it another way: what are the rates of true positives and true negatives when we test people with sniffer dogs?

      1 reply 1 retweet 2 likes
      Show this thread
    3. Health Nerd‏Verified account @GidMK 6 Jun 2018

      According to the police, the true positive rate for sniffer dogs is 80% - there is a 4 in 5 chance that an alert from a dog will result in the person having drugs or having recently been in contact with drugspic.twitter.com/4S004hPJwZ

      1 reply 1 retweet 1 like
      Show this thread
    4. Health Nerd‏Verified account @GidMK 6 Jun 2018

      So what's the true negative rate i.e. the chance that someone who DOESN'T get stopped DOES have drugs?

      1 reply 1 retweet 3 likes
      Show this thread
    5. Health Nerd‏Verified account @GidMK 6 Jun 2018

      Well, this is extremely hard to know. Some research suggests it is close to 0% (!), because huge numbers of people get into music festivals with drugs But let's assume it's better, maybe at the same 80% figure we're using abovepic.twitter.com/BzKXWM1nBl

      1 reply 1 retweet 2 likes
      Show this thread
    6. Health Nerd‏Verified account @GidMK 6 Jun 2018

      So let's say that 100 people turn up to a music festival. Of these, 5 have drugs on them and 10 have recently been in contact with drugs

      1 reply 1 retweet 1 like
      Show this thread
    7. Health Nerd‏Verified account @GidMK 6 Jun 2018

      For this type of test, we usually make a 4x4 grid to represent the rates of true and false positives, like so we can fill in the numbers at the end of the right straight uppic.twitter.com/RnDHZ9Ormf

      1 reply 1 retweet 1 like
      Show this thread
      Health Nerd‏Verified account @GidMK 6 Jun 2018

      Now, our true positive rate - taken from the police - is 80%. That means that out of our 15 "has drugs" people, we'll identify 15*0.8 = 12 correctly, and 15*0.2 = 3 incorrectly, like sopic.twitter.com/e2SFzTJ6aY

      5:49 PM - 6 Jun 2018
      • 1 Retweet
      • 1 Like
      • R.S. Lake Liz "The Mask Goes Over Your Nose AND Mouth" Ditz
      1 reply 1 retweet 1 like
        1. New conversation
        2. Health Nerd‏Verified account @GidMK 6 Jun 2018

          So far, all well and good. Lots of people correctly identified, not that many missed. Very few "false negatives" But wait, there's more

          2 replies 1 retweet 1 like
          Show this thread
        3. Health Nerd‏Verified account @GidMK 6 Jun 2018

          Now, let's apply our "true negative" rate. We're assuming that the test is 80% accurate at identifying people who don't meet our criteria. So 85*0.8 = 68 correctly identified and 85*0.2 = 17 incorrectly identified This doesn't look so good!pic.twitter.com/s3dZwKWfbp

          1 reply 1 retweet 1 like
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        4. Health Nerd‏Verified account @GidMK 6 Jun 2018

          Overall, the total number of people who test "positive" is 17+12 = 29, and the total "negative" is 3+68 = 71pic.twitter.com/LONF7eVjyI

          1 reply 1 retweet 2 likes
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        5. Health Nerd‏Verified account @GidMK 6 Jun 2018

          So using our (ENORMOUSLY GENEROUS - I'll get to this in a second) assumptions, the predictive value of the dogs is around 40%

          1 reply 1 retweet 2 likes
          Show this thread
        6. Health Nerd‏Verified account @GidMK 6 Jun 2018

          Health Nerd Retweeted Health Nerd

          This gives you some idea of the issue - for every 100 people screened by sniffer dogs, a large volume of people will be found to have drugs who don'thttps://twitter.com/GidMK/status/1004527525551173632 …

          Health Nerd added,

          Health NerdVerified account @GidMK
          So using our (ENORMOUSLY GENEROUS - I'll get to this in a second) assumptions, the predictive value of the dogs is around 40%
          Show this thread
          1 reply 1 retweet 1 like
          Show this thread
        7. Health Nerd‏Verified account @GidMK 6 Jun 2018

          In fact, even if sniffer dogs are right 80% of the time, because taking drugs is less common than not taking drugs, you'll catch more INNOCENT people than guilty ones

          1 reply 1 retweet 2 likes
          Show this thread
        8. Health Nerd‏Verified account @GidMK 6 Jun 2018

          But...what if the dogs aren't right that much? What if, as many people argue, the dogs are wrong 50% of the time? Or 75%?

          1 reply 1 retweet 1 like
          Show this thread
        9. Health Nerd‏Verified account @GidMK 6 Jun 2018

          Here's where it gets really problematic. Here's the table where the dogs are right 80% of the timepic.twitter.com/6IB2Jpy4KW

          1 reply 1 retweet 4 likes
          Show this thread
        10. Health Nerd‏Verified account @GidMK 6 Jun 2018

          And here are the tables for 50% and 25% - both of which have been observed in real lifepic.twitter.com/yOYKVtyggW

          1 reply 1 retweet 3 likes
          Show this thread
        11. Health Nerd‏Verified account @GidMK 6 Jun 2018

          Health Nerd Retweeted Health Nerd

          As you can see, the predictive power goes down significantly. Even with 50% accuracy, only 1 in 5 of the people stopped will actually have drugs on themhttps://twitter.com/GidMK/status/1004530137872809984 …

          Health Nerd added,

          Health NerdVerified account @GidMK
          And here are the tables for 50% and 25% - both of which have been observed in real life pic.twitter.com/yOYKVtyggW
          Show this thread
          1 reply 1 retweet 2 likes
          Show this thread
        12. Health Nerd‏Verified account @GidMK 6 Jun 2018

          This is why when you are testing for an uncommon outcome, your test has to be REALLY GOOD Drug sniffing dogs, even by the best estimates, are not

          1 reply 1 retweet 2 likes
          Show this thread
        13. Health Nerd‏Verified account @GidMK 6 Jun 2018

          This goes doubly for screening large numbers of people. If you screen, say, everyone who goes into a music festival, you will likely catch few offenders but identify many totally innocent people

          2 replies 1 retweet 1 like
          Show this thread
        14. Health Nerd‏Verified account @GidMK 6 Jun 2018

          So what should we do to prevent harm from drugs if we know sniffer dogs are ineffective? Tons of things. I'll make another thread about these options if anyone's interested

          1 reply 0 retweets 1 like
          Show this thread
        15. Health Nerd‏Verified account @GidMK 6 Jun 2018

          Also P.S. I got a few pieces of terminology wrong - in particular, the first tweet should read "diagnostic test" not "intervention" - and also the "25% table" is slightly misleading as I've assumed that the negative predictive power is still 75% rather than 25%

          1 reply 0 retweets 1 like
          Show this thread
        16. Health Nerd‏Verified account @GidMK 6 Jun 2018

          P.P.S TL:DR version is that drug sniffer dogs, even when they get it right 80% of the time, still make more mistakes than "hits" when not that many people have drugs

          0 replies 0 retweets 1 like
          Show this thread
        17. End of conversation

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