If 20% of the cannabis tests return false positives, that makes them almost entirely useless Why is this? Let's talk sensitivity and specificityhttps://twitter.com/DrBradMcKay/status/1131731525114839041 …
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Official figures put the rate of prosecution from drug driving at random stops at about 8%, and while it's hard to be sure it seems about half of these were for cannabis So about 4% of the people being tested almost certainly had cannabis in their system
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To work out how effective a test like this is, we use a 2x2 table like this:pic.twitter.com/CiCVlMVzmI
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Now, we know how many people actually have/don't have the result that we are interested in, so we can fill out the right column straight away:pic.twitter.com/LbkcuVZoxM
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Now, we don't know for sure what the rate of false negatives vs true positives is (sensitivity), but let's assume it's 97% - the number the police use That gives us this tablepic.twitter.com/DKPfKHFGGL
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Now, going back to the original research, we think that 20% of all the tests that should be negative are in fact positive (80% specificity) Now our table is complete!pic.twitter.com/FUlWlkRSrU
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Now, let's look at those figures Of all the negative tests, only 1/769 were wrong. That's 0.1%pic.twitter.com/dGcIE6rG6M
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However, of the tests that came back positive, only 39 out of 231 actually had cannabis in their system! That's just 17% correctpic.twitter.com/6PctjVKpLs
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What this means is that, if drug tests are wrong just 20% of the time, a positive value becomes almost meaningless
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For law enforcement, that's probably a really big problem, because they (should) know that only 1 in 7 of their arrests for drug driving are likely to be a result of recent cannabis use
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End of conversation
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