An interesting article suggests that CAPS, the government’s terrorist profiling system used in aviation security, is less effective than random screening.
Since you know if your carry-ons have been manually inspected, you’ve been questioned, you’re asked to stand in a special line, or if you’ve been frisked, you know your CAPS profile — or at least you know what you aren’t, which makes it very easy to defeat CAPS, even if the profile itself is always kept secret.