Little Brother

I put my own cloner in the pocket of my vintage black leather motocross jacket with the armored pockets and left for school. I managed to clone six tags between home and Chavez High.

It was war they wanted. It was war they'd get.

#

If you ever decide to do something as stupid as build an automatic terrorism detector, here's a math lesson you need to learn first. It's called "the paradox of the false positive," and it's a doozy.

Say you have a new disease, called SuperAIDS.

Only one in a million people gets SuperAIDS.

You develop a test for SuperAIDS that's 99 percent accurate. I mean, 99 percent of the time, it gives the correct result true if the subject is infected, and false if the subject is healthy. You give the test to a million people.

One in a million people have SuperAIDS.

One in a hundred people that you test will generate a "false positive" the test will say he has SuperAIDS even though he doesn't. That's what "99 percent accurate" means: one percent wrong.

What's one percent of one million?

1,000,000/100 = 10,000

One in a million people has SuperAIDS.

If you test a million random people, you'll probably only find one case of real SuperAIDS.

But your test won't identify one person as having SuperAIDS.

It will identify 10,000 people as having it.

Your 99 percent accurate test will perform with 99.99 percent inaccuracy.

That's the paradox of the false positive. When you try to find something really rare, your test's accuracy has to match the rarity of the thing you're looking for. If you're trying to point at a single pixel on your screen, a sharp pencil is a good pointer: the penciltip is a lot smaller (more accurate) than the pixels. But a penciltip is no good at pointing at a single atom in your screen. For that, you need a pointer a test that's one atom wide or less at the tip.

This is the paradox of the false positive, and here's how it applies to terrorism:

Terrorists are really rare. In a city of twenty million like New York, there might be one or two terrorists. Maybe ten of them at the outside. 10/20,000,000 = 0.00005 percent. One twentythousandth of a percent.

That's pretty rare all right. Now, say you've got some software that can sift through all the bankrecords, or tollpass records, or public transit records, or phonecall records in the city and catch

Cory Doctorow/Little Brother/53

terrorists 99 percent of the time.

In a pool of twenty million people, a 99 percent accurate test will identify two hundred thousand people as being terrorists. But only ten of them are terrorists. To catch ten bad guys, you have to haul in and investigate two hundred thousand innocent people.

Guess what? Terrorism tests aren't anywhere close to 99 percent accurate. More like 60 percent accurate. Even 40 percent accurate, sometimes.

What this all meant was that the Department of Homeland Security had set itself up to fail badly. They were trying to spot incredibly rare events a person is a terrorist with inaccurate systems.

Is it any wonder we were able to make such a mess?

#

I stepped out the front door whistling on a Tuesday morning one week into the Operation False Positive. I was rockin' out to some new music I'd downloaded from the Xnet the night before lots of people sent M1k3y little digital gifts to say thank you for giving them hope.

I turned onto 23d Street and carefully took the narrow stone steps cut into the side of the hill. As I descended, I passed Mr Wiener Dog. I don't know Mr Wiener Dog's real name, but I see him nearly every day, walking his three panting wiener dogs up the staircase to the little parkette. Squeezing past them all on the stairs is pretty much impossible and I always end up tangled in a leash, knocked into someone's front garden, or perched on the bumper of one of the cars parked next to the curb.

Mr Wiener Dog is clearly Someone Important, because he has a fancy watch and always wears a nice suit. I had mentally assumed that he worked down in the financial district.

Today as I brushed up against him, I triggered my arphid cloner, which was already loaded in the pocket of my leather jacket. The cloner sucked down the numbers off his creditcards and his carkeys, his passport and the hundreddollar bills in his wallet.

Even as it was doing that, it was flashing some of them with new numbers, taken from other people I'd brushed against. It was like switching the licenseplates on a bunch of cars, but invisible and instantaneous. I smiled apologetically at Mr Wiener Dog and continued down the stairs. I stopped at three of the cars long enough to swap their FasTrak tags with numbers taken offall over

cars I'd gone past the day before.