Bruce Schneier explains why making the haystack bigger doesn’t help us find a needle that keeps changing:

For example, even highly accurate medical tests are useless as diagnostic tools if the incidence of the disease is rare in the general population. Terrorist attacks are also rare, any "test" is going to result in an endless stream of false alarms.

Data Mining for Terrorists, Bruce Schneier, March 09, 2006, Schneier on Security.

And many false alarms mean a lot of innocent people being harassed needlessly, in other words, civil liberties being eroded for little or no benefit.

Bruce explains the underlying principle here, the base rate fallacy, further by referring to a formal analysis:

Floyd Rudmin, a professor at a Norwegian university, applies the mathematics of conditional probability, known as Bayes’ Theorem, to demonstrate that the NSA’s surveillance cannot successfully detect terrorists unless both the percentage of terrorists in the population and the accuracy rate of their identification are far higher than they are. He correctly concludes that "NSA’s surveillance system is useless for finding terrorists."

The surveillance is, however, useful for monitoring political opposition and stymieing the activities of those who do not believe the government’s propaganda.

Terrorists, Data Mining, and the Base Rate Fallacy, Bruce Schneier, July 10, 2006, Schneier on Security.

See Bruce’s blog entry or Prof. Rudmin’s analysis for the details, which amount to there simply aren’t enough terrorist plots to find by this kind of method.

What should we do, then? As Bruce said in his previous post on this subject:

Finding terrorism plots is not a problem that lends itself to data mining. It’s a needle-in-a-haystack problem, and throwing more hay on the pile doesn’t make that problem any easier. We’d be far better off putting people in charge of investigating potential plots and letting them direct the computers, instead of putting the computers in charge and letting them decide who should be investigated.

Or, as I noted in Liberty Is Security, research shows that one of the best ways to reduce terrorism is to increase civil liberties of countries that breed terrorist groups. Given that, why would reducing civil liberties in the U.S. in the name of spotting sleeper terrorist cells be a good idea, especially when the method proposed doesn’t even work?

Let me refer once again to an even earlier thinker on this subject:

Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety.

–Benjamin Franklin, An Historical Review of the Constitution and Government of Pennsylvania, 1759

Liberty is security.

-jsq