For the sake of argument, let us say one out of every million people is a terrorist (an overestimate). Let us also imagine there exists a machine that can determine whether someone is a terrorist with 99.9 percent accuracy.
What are the odds an individual tested is a terrorist? 0.1 percent chance right?
So the 99.9 percent accurate test will give you the wrong answer 99.9 percent of the time. This is what is known as the false positive paradox, and it completely dismantles any possible justification for the NSA’s dragnet surveillance of our phone calls and emails.