Background checks ( of the TSA's depth ) don't reveal if a candidate is susceptible to blackmail or coercion, so physical checks are always a good idea.
After all it's not who you are but what you possess that constitutes a threat.
So what is the point of pre-"clearance"? Revenue, I suspect.
Then why have a background check if it's ineffective?
1 check - x% effective.
2 checks - (x + y)% effective.
n checks - (x + y + ...[n times]) % effective.
That's the issue with security theater.
I'm not sure why people here keep referring to everything as security theater, to be honest.
Yes, for the purpose of "protecting American at large", it's approximately that.
And it's very simple to see too! The most effective way to prevent airplane terror is to stop everyone from traveling, are we gonna be safer thank to that, even disregard all the downsides of not being to travel? You know the bad guy can just plot their scheme elsewhere, right? What's next? Millimeters scanner at Taylor Swift concert?
Also, since this is HN, I believe your maths is incorrect. The effective rate is (100 - (100-x)(100-y)(100-z)...)%
Further, it's theatric because the many of the "dangerous" items have no obvious malicious use, and the rules are totally arbitrary. I can't bring knitting gear on an airplane, but my small pocket knife is totally fine (according to official rules, though some airport security agents will disagree).
Finally, any attempt at plane hijacking nowadays will cause passengers to fight for their life to prevent it. Prior to 9/11 people would just sit still and wait to land in Cuba or whatever, but now people associate hijacking with flying into a building and will act thereafter.
It could actually be safer to allow knives and similar "weapons" that can't penetrate the plane body, so people have a chance to defend themselves when some terrorist inevitably smuggles aboard a handgun and starts shooting.
There are a lot of "weapons" you could take based on the Tools category [1].
[0] https://www.tsa.gov/travel/security-screening/prohibited-ite... [1] https://www.tsa.gov/travel/security-screening/prohibited-ite...
0.000007656967% of the time they did something. In reality it's half of that. In reality of actual danger being present it's even lower than half of that. I'm talking near 7.6e^-8 in rarity.
The odds of being stuck by lightning is higher than the TSA ensuring anyone's safety.
[0] http://blog.tsa.gov/2015/01/tsa-2014-year-in-review.html
I'm Canadian, but I have a similar story. I was travelling with my mother, and my backpack was swabbed and X-Rayed at security. No problems. It wasn't until I got home and was unpacking that I realized I had a bottle of aerosol nitroglycerine (!) in my carry-on! My mother's got a heart condition and I was carrying around a spare canister just in case something happened.