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> https://en.wikipedia.org/wiki/Casio_F-91W https://en.wikipedia.org/wiki/Casio_F-91W According to secret documents issued to interrogators at Guantánamo Bay,

by qb45 11y ago

> https://en.wikipedia.org/wiki/Casio_F-91W https://en.wikipedia.org/wiki/Casio_F-91W According to secret documents issued to interrogators at Guantánamo Bay, obtained and released by The Guardian, "the Casio F-91W digital watch was declared to be 'the sign of al-Qaeda' and a contributing factor to continued detention of prisoners by the analysts stationed at Guantánamo Bay. Briefing documents used to train staff in assessing the threat level of new detainees advise that possession of the F91W – available online for as little as £4 – suggests the wearer has been trained in bomb making by al-Qaeda in Afghanistan." United States Military intelligence officials have identified the F91W as a watch that terrorists use when constructing time bombs. One learns something new every day. Good that I went with the W59 instead :)

cmdkeen 11y ago
The irony is that a huge number of people in the UK military wear the F91W as it does everything you need a watch to do, is dirt cheap and hard to break. Don't know if the same applies in the US, but I wouldn't put it past the prospect that at least one guard at Guantanamo wears the same watch...
pjc50 11y ago
This does sound like a classic example of someone in the intelligence service being bad at conditional probability. "All the people we captured wear F91Ws" does not imply "everyone wearing an F91W is a terrorist"!
- qb45 11y ago
  Actually, they say they found time bomb detonators based on this watch. United States Military intelligence officials have identified the F91W as a watch that terrorists use when constructing time bombs. https://en.wikipedia.org/wiki/File:Al_Qaida_watch_timer_on_perf_board_84071_-abc.jpg https://en.wikipedia.org/wiki/File:Al_Qaida_watch_timer_on_p...
  - ska 11y ago
    You're missing the point about conditional probability. P(has a F-91W | is a terrorist) != P(is a terrorist | F-91W) Via Bayes theorem you can fix this with a ratio P(is a terrorist)/P(has a F-91W) So if the global population of people with F-91W is large relative to the population of terrorists, you are going to have a lot of false positives. e.g. say that 100% of terrorists have a F-91W, that there are 10,000 terrorists, and that 100M people have F-91W then you still only have a .01% chance that a given F-91W wearer is a terrorist. Obviously there is more information to go on than just a watch, but that's the point you were replying to.
    - qb45 11y ago
      And you are missing the point about other reasons to suspect F91W users, which have nothing to do with counting watches worn by known terrorists. Also, you probably don't know whether this watch actually was common in Afganistan in early 2000s.
      - ska 11y ago
        No, I am not. That was the purpose of: "Obviously there is more information to go on than just a watch, but that's the point you were replying to." Your reply did not, in any way, address the conditional probability issue. Which is important, although clearly oversimplified in my example. This has nothing particular to do with terrorist detection, but to do with detection of any sort where the true positive rate is low (e.g. cancer screening, drug detection,etc.). This is often a very real problem in practical application, and saying "but they actually really found X" does not address it at all. It is exactly that sort of muddy thinking that causes problems with detection in application. Obviously this is manageable by including enough different information that the marginals get much tighter - if you can find the right ones - but that wasn't the point being made. A cogent reply would have pointed out how exactly additional information was used to reduce this risk.
        qb45 11y ago
        There is a possibility that you reading too much into the comment which started this thread. It appears to have quite literally meant "the CIA caught terrorists with X so they figured everyone with X is a terrorist". Without convincing evidence that: a) this was the reasoning used b) this reasoning was invalid > Your reply did not, in any way, address the conditional probability issue. I think I did it here: Also, you probably don't know whether this watch actually was common in Afganistan in early 2000s.
- pbhjpbhj 11y ago
  But it does mean that scrutinising people with f91w watches should increase detection rates.
  - ska 11y ago
    Sure, and I'm not saying you are wrong. The point was that sensitivity and specificity are always a trade off. Failing to understand this correctly has been a huge problem in practice. Examining everyone who breathes, after all, has very high sensitivity.
  - Someone 11y ago
    But that may only be because the terrorists wearing those watches are worse at not being found out than the ones not wearing them.
- coldtea 11y ago
  Why, did you hear anyone state that? Only that "wearing F91Ws" increases the chances of them being a terrorist, which is already pretty high, given that we're talking about "detainees", not random people.

by qb45 11y ago

cmdkeen 11y ago

The irony is that a huge number of people in the UK military wear the F91W as it does everything you need a watch to do, is dirt cheap and hard to break. Don't know if the same applies in the US, but I wouldn't put it past the prospect that at least one guard at Guantanamo wears the same watch...

pjc50 11y ago

This does sound like a classic example of someone in the intelligence service being bad at conditional probability. "All the people we captured wear F91Ws" does not imply "everyone wearing an F91W is a terrorist"!

qb45 11y ago
Actually, they say they found time bomb detonators based on this watch. United States Military intelligence officials have identified the F91W as a watch that terrorists use when constructing time bombs. https://en.wikipedia.org/wiki/File:Al_Qaida_watch_timer_on_perf_board_84071_-abc.jpg https://en.wikipedia.org/wiki/File:Al_Qaida_watch_timer_on_p...
- ska 11y ago
  You're missing the point about conditional probability. P(has a F-91W | is a terrorist) != P(is a terrorist | F-91W) Via Bayes theorem you can fix this with a ratio P(is a terrorist)/P(has a F-91W) So if the global population of people with F-91W is large relative to the population of terrorists, you are going to have a lot of false positives. e.g. say that 100% of terrorists have a F-91W, that there are 10,000 terrorists, and that 100M people have F-91W then you still only have a .01% chance that a given F-91W wearer is a terrorist. Obviously there is more information to go on than just a watch, but that's the point you were replying to.
  - qb45 11y ago
    And you are missing the point about other reasons to suspect F91W users, which have nothing to do with counting watches worn by known terrorists. Also, you probably don't know whether this watch actually was common in Afganistan in early 2000s.
    - ska 11y ago
      No, I am not. That was the purpose of: "Obviously there is more information to go on than just a watch, but that's the point you were replying to." Your reply did not, in any way, address the conditional probability issue. Which is important, although clearly oversimplified in my example. This has nothing particular to do with terrorist detection, but to do with detection of any sort where the true positive rate is low (e.g. cancer screening, drug detection,etc.). This is often a very real problem in practical application, and saying "but they actually really found X" does not address it at all. It is exactly that sort of muddy thinking that causes problems with detection in application. Obviously this is manageable by including enough different information that the marginals get much tighter - if you can find the right ones - but that wasn't the point being made. A cogent reply would have pointed out how exactly additional information was used to reduce this risk.
      - qb45 11y ago
        There is a possibility that you reading too much into the comment which started this thread. It appears to have quite literally meant "the CIA caught terrorists with X so they figured everyone with X is a terrorist". Without convincing evidence that: a) this was the reasoning used b) this reasoning was invalid > Your reply did not, in any way, address the conditional probability issue. I think I did it here: Also, you probably don't know whether this watch actually was common in Afganistan in early 2000s.
pbhjpbhj 11y ago
But it does mean that scrutinising people with f91w watches should increase detection rates.
- ska 11y ago
  Sure, and I'm not saying you are wrong. The point was that sensitivity and specificity are always a trade off. Failing to understand this correctly has been a huge problem in practice. Examining everyone who breathes, after all, has very high sensitivity.
- Someone 11y ago
  But that may only be because the terrorists wearing those watches are worse at not being found out than the ones not wearing them.
coldtea 11y ago
Why, did you hear anyone state that? Only that "wearing F91Ws" increases the chances of them being a terrorist, which is already pretty high, given that we're talking about "detainees", not random people.