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
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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 …
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
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
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
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%
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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