3 ms·

> E.g. if a given test were to generate a false positive, you would expect that it wouldn't generate a false positive the next time if run on the same article,

by jlgreco 13y ago

> E.g. if a given test were to generate a false positive, you would expect that it wouldn't generate a false positive the next time if run on the same article, especially if the machine correctly doesn't alert on other innocuous articles. Well yes, I would. Some false positives can be transient, others are not. If I just came from the shooting range I would not be surprised if I got a non-transient false-positive. Now that is an obvious case and I would immediately tell them that I had been to the shooting range, likely resolving the issue, but it is just a simple example of a false positive that is not transient. How common is a non-transient false-positive where the victim doesn't immediately have a good idea what the cause is? Well, there are several reports from HN users in this thread that describe situations in which it could allegedly occur. One cites hand lotion as a potential cause of non-transient false-positives; luckily for his wife the agents volunteered that hypothesis so she wasn't left guessing. The author of the article supposes that an over-the-counter chemical was the cause of his non-transient false positive; it's not like he was a lab tech working with synthesized stuff that nobody else ever comes into contact with. Unless the machines are shit (a distinct possibility), I would expect transient false-positives to be relatively infrequent while non-transient false-positives would be reliably and regularly caused by a wide range of substances that share chemical properties with known explosives. Most of these non-transient false positives are likely quickly resolved without much ado. His was not. Edit: Perhaps the real problem here is with the terminology. These machines are not really bomb detectors, or even explosive detectors. They are chemical detectors. Calling them bomb or explosive detectors is like calling a metal detector a "gun detector". Sure, finding those things may be why it is there, but that is not actually what it does.

timv 13y ago
> Well, there are several reports from HN users in this thread that describe situations in which it could allegedly occur Which makes the detectors useless as an interrogation tool. If the alarm sounds, then check the person for explosives. That makes perfect sense, it's a useful tool for finding explosives on people. But if you cannot find explosives on the person, what do you do then? Any even slightly training terrorist is going to know what other products would produce the same detection result as the bomb they just built. So they'll pretend to think for a while, and then say "I work in a supermarket, and a customer dropped hand lotion on the floor this morning, and I had to wipe it up... can that set off your machine?" It seems like there's a magic answer you can give that will let you go free, you just need to know the right thing to say. If the investigator likes you (i.e. thinks you're probably not a terrorist) they'll give you hints about what you should say. If they don't then you're on your own. Someone who can give the right answer is either: * Good at analysis, so they can make a good guess of what might be setting of the machine. * Someone who's been through this before * Someone who got a friendly hint from the investigator * A not-so-dumb terrorist Someone who can't give the answer is either: * A normal person * A dumb terrorist Given the low prevalence of terrorists, that would be the least likely explanation in either scenario, so the whole line of questioning is pretty much useless, except as a way of applying pressure to someone who you've decided is worth applying pressure to. When that decision is based on some genuine piece of evidence, then it might be a legitimate law enforcement technique. When it's based on the gut-feel of the officers in question, it becomes a front for racial profiling.

by jlgreco 13y ago

timv 13y ago

> Well, there are several reports from HN users in this thread that describe situations in which it could allegedly occur Which makes the detectors useless as an interrogation tool. If the alarm sounds, then check the person for explosives. That makes perfect sense, it's a useful tool for finding explosives on people. But if you cannot find explosives on the person, what do you do then? Any even slightly training terrorist is going to know what other products would produce the same detection result as the bomb they just built. So they'll pretend to think for a while, and then say "I work in a supermarket, and a customer dropped hand lotion on the floor this morning, and I had to wipe it up... can that set off your machine?" It seems like there's a magic answer you can give that will let you go free, you just need to know the right thing to say. If the investigator likes you (i.e. thinks you're probably not a terrorist) they'll give you hints about what you should say. If they don't then you're on your own. Someone who can give the right answer is either: * Good at analysis, so they can make a good guess of what might be setting of the machine. * Someone who's been through this before * Someone who got a friendly hint from the investigator * A not-so-dumb terrorist Someone who can't give the answer is either: * A normal person * A dumb terrorist Given the low prevalence of terrorists, that would be the least likely explanation in either scenario, so the whole line of questioning is pretty much useless, except as a way of applying pressure to someone who you've decided is worth applying pressure to. When that decision is based on some genuine piece of evidence, then it might be a legitimate law enforcement technique. When it's based on the gut-feel of the officers in question, it becomes a front for racial profiling.