Overall, the total number of people who test "positive" is 17+12 = 29, and the total "negative" is 3+68 = 71pic.twitter.com/LONF7eVjyI
Epidemiologist. Writer (Guardian, Observer etc). "Well known research trouble-maker". PhDing at @UoW Host of @senscipod Email gidmk.healthnerd@gmail.com he/him
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Overall, the total number of people who test "positive" is 17+12 = 29, and the total "negative" is 3+68 = 71pic.twitter.com/LONF7eVjyI
So using our (ENORMOUSLY GENEROUS - I'll get to this in a second) assumptions, the predictive value of the dogs is around 40%
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,
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
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%?
Here's where it gets really problematic. Here's the table where the dogs are right 80% of the timepic.twitter.com/6IB2Jpy4KW
And here are the tables for 50% and 25% - both of which have been observed in real lifepic.twitter.com/yOYKVtyggW
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,
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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