Now let's say that law enforcement apprehends a person by using this technology. Given these assumptions, you might guess that the person would be almost certain to commit a terrorist act. Right? Well, no. Even with the system's amazing data-mining powers, there is only a tiny probability the apprehended person will go on to become an active terrorist.
To see why this is so and to make the calculations easy, let's postulate a population of 300 million people of whom 1,000 are future terrorists. The system will correctly identify, we're assuming, 99 percent of these 1,000 people as future terrorists. Thus, since 99 percent of 1,000 is 990, the system will apprehend 990 future terrorists. Great! They'll be locked up somewhere.
But wait. There are, by assumption, 299,999,000 non-terrorists in our population and the system will be right about 99 percent of them as well. Another way of saying this is that it will be wrong about 1 percent of these people. Since 1 percent of 299,999,000 equals 2,999,990, the system will swoop down on these 2,999,990 innocent people as well as on the 990 guilty ones, incarcerating them all.
That is, the system will arrest almost 3 million innocent people, about 3,000 times the number of guilty ones. And that occurs, remember, only because we're assuming the system has these amazing powers of discernment! If its powers are anything like our present miserable predictive capacities, an even greater percentage of those arrested will be innocent.
Of course, this is a fiction and the numbers, percentages and assumptions are open to very serious question. Nevertheless, the fact remains that since almost all people are innocent, the overwhelming majority of the people rounded up using any set of reasonable criteria will be innocent. And even though the system proposes only increased scrutiny rather than incarceration for suspected pre-perpetrators, such scrutiny might very well lead over time to a detailed government dossier on each of us with little if any increase in security.
There are many ways to combat terrorism without entering a futuristic Wonderland devoid of privacy rights.
Professor of mathematics at Temple University and adjunct professor of journalism at Columbia University, John Allen Paulos is the author of several best-selling books, including Innumeracy, and the forthcoming A Mathematician Plays the Market, which will be published in the spring. His Who’s Counting? column on ABCNEWS.com appears the first weekend of every month.