Hi Readers — Why was I up till 12:45 last night? I HAD to finish, “Little Brother.” It’s the young adult book by boingboing’s Cory Doctorow that’s all about what would happen after a terrorist attack if the government started suspecting EVERYONE of terrorism, and most of the people were fine with this.
Naturally, the hero is a geeky/brave 17-year-old and his posse of smart friends, and the action is non-stop. Â Naturally, i’ts Â being made into a movie. UNnaturally, I loved it. Â Normally, I’m more of a historical fiction kind of gal — think, “Girl with a Pearl Earring” — Â so this was a book I had to filch from my 14-year-old (who is mortified I read one of his favorite things).
In the most exciting way, the book makes you question all the security measures we take for granted: Are they really making us safer? Are they maybe making us LESS safe? Better still, it explains so many of the issues I’m always grappling with. Like — you know how I find the Sex Offender Registry disturbing because so many of the people on it don’t pose a threat to children? And you know how I’m also upset at the idea of background checks for anyone who even walks into a school, a practice that’s becoming more Â and more common? I want our kids to be safe, too. So why should these things bother me? What’s the downside, besides the occasional bureaucratic mix up?
Well here’s how Doctorow’s hero, Marcus, explains the problem of casting too wide a net when searching for evil:
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 Super-AIDS. Only one in a million people gets Super-AIDS. You develop a test for Super-AIDS 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 Super-AIDS. One in a hundred people that you test will generate a “false positive” — the test will say he has Super-AIDS 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 Super-AIDS. If you test a million random people, you’ll probably only find one case of real Super-AIDS. But your test won’t identify *one* person as having Super-AIDS. 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 pencil-tip is a lot smaller (more accurate) than the pixels. But a pencil-tip 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 twenty-thousandth of a percent.
That’s pretty rare all right. Now, say you’ve got some software that can sift through all the bank-records, or toll-pass records, or public transit records, or phone-call records in the city and catch 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.
That’s such an easy-to-understand explanation of what can happen when we start suspecting too many people of any kind of evil. And rest assured, one of the innocents pulled into the vortex of “Suspected Bad Guy” is our funny, hacking (and horny) “Little Brother” hero, Marcus. Will he get out? Will he change the course of history? Will he get the cute girl with glasses?
There’s only one way to find out! (At least until they make the movie.) — Lenore
P.S. I am not blocking anyone’s comments. My WordPress filter may be doing it. I can’t even find the “missing” comments in my “Awaiting Moderation” cue. So I’m mystified and sorry. Anyway, here’s a link to a free download of the book:Â http://craphound.com/littlebrother/download/