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