Spiff,

Your comment that explosives-detection machines are wrong "almost" as often as they are right is too generous! Consider the following analysis:

Let's suppose that these machines detect 99% of explosives in bags and report false alarms only 1% of the time. Then, let's assume that one bag in a thousand contains explosives (obviously this is way too high, but let's use it to illustrate my point).

Then, for every 1,000,000 bags passing through, the machines will correctly detect 990 of the expected 1000 bombs. (They will miss 10 bombs, but that's another issue entirely!) They will also falsely report 9990 bombs in the 999,000 bomb-free bags. In other words, false alarms will outnumber legitimate alarms by slightly more than 10 to 1!

Real life is even worse than this illustration. The number of real bombs is, of course, much less than one in a thousand. And the percentage of false alarms is much higher than 1% (some say 5%). When the vast majority of alarms are found to be false -- as is inevitable -- the security personnel, being only human, will eventually ignore them.

What a complete waste of time and money.

Bruce