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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 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
      Show this thread
    2. 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
    3. 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
    4. 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
    5. 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
    6. 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
    7. 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
    8. 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
    9. 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
    10. 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
      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

      6:37 PM - 6 Jun 2018
      • 1 Like
      • C H - fully vaxxed 💉💉
      1 reply 0 retweets 1 like
        1. New conversation
        2. 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
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        3. 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
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        4. End of conversation

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