In general it should go without saying that I agree with PZ, unless stated otherwise. I just want to add a little something from the perspective of a computer science nerd whose been around a bit with the notion of data mining. I also want to prove that I didn’t go to grad school for nothing. (It cost me thousands! <drum fill>)
In data mining, we have the twin problems of “false positives” and “false negatives.” For example, suppose there is a certain disease that is very rare in the population, but very deadly if not detected. Let’s say that one person in a million has this disease, which is 0.0001%. If we devise a screening procedure for this disease, we would like to be very sure that we catch almost all people who are at risk.
So let’s say we apply this screening test to somebody, and they (unbeknownst to you) have the disease, but your test says they do not. That is a false negative, and it can kill the person, since they won’t be treated.
Let’s say we apply the screening test to somebody who does not have the disease, and the test says that they have it. That is a false positive. But the consequences of a false positive are not as dire. If the screen says you have the disease, you follow up with another test that is more rigorous, and more expensive, to prove for certain that you need treatment.
As I said, we would like to prevent false negatives as much as we can, so we set the sensitivity of the screen to be very high. Even so, the test will give a positive result for only one person for every thousand who takes the test, and those few people who have the disease are almost certain to get a positive.
Okay, now let’s say I go to the doctor and I get a positive result. How worried should I be that I have the disease? As it turns out, not very worried at all.
See, the screening test says I have the disease, but I almost certainly don’t. One in a thousand people gets a positive. One in a million people actually has the disease. Therefore, only one in a thousand positive tests are of people who actually have the disease.
Is that clear? Good. Now let’s consider terrorists…
|Pop quiz: which of these two guys is the terrorist? Answers at the bottom of the post, or you can mouse over the pictures for more information.|
How many terrorists are there in the US? I don’t know, but let’s look at it this way. Statistics show that about five people per hundred thousand are the victim of a homicide each year. The vast majority of murderers aren’t actually terrorists as such, but let’s be generous and assume that one in ten is. That means that at any given time, 5 in a million are likely to be terrorists, or 0.0005%.
Suppose we start profiling people who simply look Middle Eastern, like mug shot number two up there. There are about 1.5 million Arab Americans living in the US, which is 0.5% of all people here. Similarly, there are 2.5 million people who are practicing Muslims, which accounts for 0.8% of all people living here.
That means — even if we assume that only Muslims and people who “look Muslim” are terrorists (which is clearly not true; see quiz answers) and even if we use the fairly high number I guesstimated as terrorists — if you round up all the people who “fit the profile” then, similar to the disease, fewer than one in a thousand are likely to be actual terrorists.
But unlike performing a screen for a disease that might return a false positive, the cost of falsely accusing a guy of terrorism is not negligible. We’re saying that these people deserve to be harassed, frisked, searched, delayed, pegged as a security threat. Not just once, but every time they dare to travel. We’re also saying that we need to maintain a staff of thousands of paranoid security officials to do a job which is, in the long run, not as effective as assigning detectives to follow up on reports of actual, credible threats that go beyond “He don’t look American enough.” There is such a thing as civil liberties, and it’s not a pointless question to ask whether this is a good place for us to be going.
Pop Quiz answers
- Left: Anders Breivik, currently on trial for the murder of 69 77 people, mostly teenagers.
- Right: Comedian Kumail Nanjiani, wanted for slaying thousands of audiences with his hilarious wit.
Update: Greg Laden also has an excellent response, which I almost dismissed by mistake due to his cleverly satirical introduction.