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Old Oct 1, 16, 12:23 am
Join Date: Sep 2015
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Originally Posted by Zorak View Post
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 View Post
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.
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