Originally Posted by
Zorak
Is the WTMD sensitivity adjustable, and/or variable by site/equipment? I have a watch with a metal body and band; after being selected randomly twice in a row out of Precheck at SFO (after having never been randomly selected), I started taking the watch off along with emptying my pockets and have not been randomly selected since.
I definitely do not consider this statistically significant, just wondering if it's a possible cause.
I think they are adjustable - or at least they can vary between countries: I've got a belt which has mostly been ok in UK airports, but has alarmed a few times in mainland Europe. In the US it seems to guarantee me an alarm (it alarmed the first 2 times I used precheck, since then I have always removed it, and I've worn it once recently to test again - but departing from LHR recently I didn't have issues).
FWIW there are two separate indications: there's metal being deteted, and there's a random selection. They make different tones. At SEA recently I went through, had the (metal) beep, went back and took off my belt, went through again: different tone, the agent emphasised that this was a random (and told the second agent manning the AIT the same thing).
Originally Posted by
ehallison
We don't know the chances of being selected, but it's much less than 50/50. I'm guessing about a 10% rate (which seems within the realm of possibility based on my experience, and my husband's previous experience). My probability calculating skills are a little rusty, but if we assume 1/10 chance of being chosen, then the chances of being chosen 7 out of 8 random tries are miniscule: .00001% or 1 in 100 thousandths percent. (1/10 7 times, times 9/10)
So I'm still skeptical that it's random.
Yes, that's a perfect example of randomness.
Note, I don't think your combinatorics are quite correct. You'd probably want to multiply that by 8, since the non-selected event could have happened at any of 8 positions, i.e. 8 permutations. But it's a bit late for me so I might be being stupid right now.