NSA and logic

On Bob Edward's Weekend, they have someone saying we don't need the NSA because there have not been many deaths from terrorists in the U.S. in recent years. I don't know if the NSA has been effective, but the fact that we haven't had much problem with terrorism in recent years is hardly evidence that they are ineffective.

The same sort of illogic that arises whenever actions against a problem are effective, like vaccinations.