<i>Reason</i> Rounds

Kerry Howley talks about medical paternalism and prescriptioin drugs:

There are perks to being governed by an insane military dictator. Creating a whole system of repression is a big job for one general, and anarchy bubbles to the surface in the empty spaces totalitarianism has yet to fill. Last year, while living in the Southeast Asian nation of Myanmar, my phones were tapped, my journals were read, my work was censored, and for the first time in my life, I was given the authority to care for my own body.

There is no prescription drug system in Myanmar, but there are plenty of illnesses waiting to befall an effete Western immune system. My expatriate colleagues and I were free to treat our ailments as we saw fit. We staved off food poisoning and bouts of malaria with frequent trips to the local pharmacy, consulting doctors when necessary, but ultimately responsible for our own medical decisions. We formed doctor-patient relationships that were partnerships rather than paternalistic hierarchies, and each of us lived to tell the story.

Also, Julian Sanchez demonstates the difference between test sensitivity and positive predicitive value with regard to drug-sniffing dogs.

Let's grant that the dog is 95 percent accurate. Now, you might think that sounds pretty good—95 percent certainty would surely count as probable cause, right? The problem is making the error—and I wonder whether maybe the justices did this—of inferring from a 95 percent accuracy rate that you're only going to end up physically searching one innocent person for every 19 who really do have drugs. But if searches are indiscriminate, that's wrong, because the vast majority of motorists won't have drugs.

Assume, and I suspect this is a big overestimate, that one in 100 motorists are driving around with drugs. If you're sweeping them randomly, a 95 percent accurate dog is going to "alert" for five innocent people for every one it catches with actual drugs. And those five people are going to get their cars torn apart. If the dogs are less accurate, it'll be many more than that. Anyone think that passes Fourth Amendment muster?

This is a well-understood problem in medicine. To determine the accuracy of a given diagnostic test, sensitivity and specificity are calculated. Sensitivity is the probability that an individual with disease X will have positive test X. (Specificity, meanwhile is the inverse of the probabilty of a person without disease X will test negative. They are usually quited in tandem, but given the nature of the discussion, I assume that we're talking about accuracy of postive tests - there is no one number for test "accuracy," more like a number for the accuracy of positive tests and a separate number for the accuracy of negative tests.)

However, to determine the clinical significance of a test, the positive and negative predictive values are more valuable. PPV is the probability of having a disease with a positive test. A test may be insanely accurate, but if a disease is so rare, the clinical significance of a positive test may be next to nothing.

The point is that more random (in the sense that searches, now that the SCOTUS ruled dog sniffs constitutional, will be more random than they were before) searches of a relatively rare event (drugs in cars) will lead to a whole bunch of "probable cause" which is, in truth, very improbable.

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