> Let's look at some numbers. We'll be optimistic -- we'll assume the system has a one in 100 false-positive rate (99 percent accurate), and a one in 1,000 false-negative rate (99.9 percent accurate). Assume 1 trillion possible indicators to sift through: that's about 10 events -- e-mails, phone calls, purchases, web destinations, whatever -- per person in the United States per day. Also assume that 10 of them are actually terrorists plotting.

This unrealistically accurate system will generate 1 billion false alarms for every real terrorist plot it uncovers. Every day of every year, the police will have to investigate 27 million potential plots in order to find the one real terrorist plot per month. Raise that false-positive accuracy to an absurd 99.9999 percent and you're still chasing 2,750 false alarms per day -- but that will inevitably raise your false negatives, and you're going to miss some of those 10 real plots.

This isn't anything new. In statistics, it's called the "base rate fallacy," and it applies in other domains as well. 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.

This is exactly the sort of thing we saw with the NSA's eavesdropping program: the New York Times reported that the computers spat out thousands of tips per month. Every one of them turned out to be a false alarm.

And the cost was enormous -- not just for the FBI agents running around chasing dead-end leads instead of doing things that might actually make us safer, but also the cost in civil liberties. The fundamental freedoms that make our country the envy of the world are valuable, and not something that we should throw away lightly.

https://www.schneier.com/essays/archives/2005/03/why_data_mi...

And if you're trying to prevent atrocities, you'll convict them before they've done anything wrong.

You also need to realize that people that were doing unpleasant surveillance in parked cars, at night, trying to figure out what a known arms dealer was up to, are now enjoying the comfort of a desk, looking at a database of intercepted SMSes that don't make any sense, and waiting for the shift to end.

There is also another case which is finding out who did it - and in recent reports of police investigations there is always about use of surveillance cameras. The surety of being caught is also important in prevention. Maybe less in the case of suicide terrorists - but they are really a minority of all crime.

I have the feeling that people are so much against surveillance that they cannot stand any analysis of the arguments used. This is counterproductive.

Have you watched the news these last few days? Known extremists managed to get AK-47s and explosives in the middle of Paris, at the highest alert level of a military-on-the-streets Vigipirate program, while the borders were closed and the city's security was further elevated for an upcoming international meeting about climate.

They were known to law enforcement, they communicated over cleartext intercepted channels, they lost themselves in the noise of tens of millions of people joyfully filling some Stasi's storage units with their private conversations.

And all this less than a year since a similar event lead to the incompetents asking for even more data that they can stash for after the fact investigations. So there's a valid use: finding out, in retrospect, exactly how they failed to do their jobs. And maybe ask for even more funding, even more privacy invasions, even more power.