Bad Positives

Great article by Cory Doctorow in the Guardian on terrorism and statistics. He explains in simple terms how we're spending enormous amounts of money on threats that are statistically highly unlikely, instead of focusing on unimportant things like, say, poverty, climate change or the world food crisis – which are all vastly more likely to kill you than a terrorist attack.

I mean, I understand the idea of the military: significant proportions of our state's populations die from warfare, let's make a strong military to prevent that from happening. The logic is flawed (militaries prevent wars, but they also cause them), but I understand where it's coming from. The idea of huge anti-terrorism departments like the US Dept of Homeland Security spending billions of dollars on things which are likely to kill very few people is pretty ridiculous.

Especially when you have a proper understanding of the paradox of the false positive:

Our innumeracy means that our fight against these super-rarities is likewise ineffective. Statisticians speak of something called the Paradox of the False Positive. Here's how that works: imagine that you've got a disease that strikes one in a million people, and a test for the disease that's 99% accurate. You administer the test to a million people, and it will be positive for around 10,000 of them – because for every hundred people, it will be wrong once (that's what 99% accurate means). Yet, statistically, we know that there's only one infected person in the entire sample. That means that your "99% accurate" test is wrong 9,999 times out of 10,000!

Terrorism is a lot less common than one in a million and automated "tests" for terrorism – data-mined conclusions drawn from transactions, Oyster cards, bank transfers, travel schedules, etc – are a lot less accurate than 99%. That means practically every person who is branded a terrorist by our data-mining efforts is innocent.

In other words, in the effort to find the terrorist needles in our haystacks, we're just making much bigger haystacks.

Go and read the whole thing. Essential reading for the modern boy and girl who wants to get ahead in the world.

The paradox of the false positive is also the rather unlikely theme of Doctorow's young adult novel Little Brother, which you can download for free here. And by the way, giving the book away free was such a bad idea that the book debuted at #9 on the NY Times bestseller list. Man, these copyright thieves with their file sharing.