It's the Math

By David Chandler

"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized." - Fourth Amendment to the Constitution of the United States.

What's the big deal about probable cause? Why not just screen everyone's phone calls and email? Aren't we serious about catching "the terrorists"?

Screening everyone for "terrorism indicators" is a bad idea for the same reason it is a bad idea to screen everyone for drugs or vaccinate everyone for smallpox. These may seem counterintuitive to a morally scrupulous, insecure general public but it's an example of how mathematics weasels its way into public affairs.

A big problem in mass screenings is false positives. Say you are screening 297,991,131 people (the current population of the United States) trying to find 1000 potential terrorists. Say the criterion for screening gives a false positive reading 1% of the time. That means you are affecting the lives (arresting, falsely accusing, denying air travel to, deporting, or generally making life miserable for) nearly 3 million innocent people in order to try to catch 1000 people you think may have evil intentions. Meanwhile, the actual terrorists are undoubtedly carrying false ID, avoid discussing their plans in email, and quite possibly have friends in high places, so they walk onto airplanes and come in and out of the country, and make their plans unimpeded. All of this is even assuming no one is playing politics with the screening criteria...a major bad assumption, as a quick Google search on "COINTELPRO" would show.

The same mathematics explains why they don't vaccinate the whole population for smallpox anymore. Since the smallpox virus is all but extinct (except in bioweapons labs) the chance of catching smallpox is very slim, even with the possibility that some nut may start sending out letters laced with the virus. On the other hand some people would die from a smallpox vaccination. The danger of dying from the vaccination is small, percentage-wise, but a small percent of a large population means you would be killing a lot of people with a massive vaccination program...a lot more than were killed in the anthrax attacks, for instance. Mass vaccinations make sense if the potential numbers of disease victims is large. Otherwise they can do more damage than good.

Screening large populations for small numbers of targets with screening techniques that can yield even a small percentage of false positives is bad practice, mathematically. It doesn't work well regardless of the emotional appeal of the issue involved. The health community figured all this out long ago.

A totalitarian government, of course, doesn't care how many innocent people are hurt along the way, which is why military checkpoints on roads are a standard cinematic technique to symbolize totalitarian rule. Our society isn't supposed to be that way. That's why the authors of the Bill of Rights insisted on "Probable Cause" as the standard before the government should be allowed to ransack people's homes or belongings (or phone conversations or email).

The medical profession should share its wisdom with the administration. More to the point, we should learn the math ourselves and use it to stand up to the administration, which obviously doesn't care how many innocent people are hurt along the way.

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