One other thing I've learned about counterfeiters is that there is almost no product too cheap to counterfeit. If there is a suitable demand for the product, there will be counterfeits. You can see this in the cheap USB AC adapters above, but also in items like the Casio F-91W[1], a cheap retro watch that has made a comeback in recent years.

Search on eBay and you'll find tons of them, mostly for really cheap prices, i.e. < $10 or even < $5. It's virtually impossible to identify a fake on the auction site, and even when you first have it it's difficult to tell. (Here[2] is one test, and a teardown comparison between fake vs. real) So, just assume anything super-cheap is fake, and you won't be let down.

However, there are some real differences. You'll notice that the band might look/feel slightly cheaper, and most importantly, the thing doesn't keep time. I have a legit F-91W, and for the most part, the watch doesn't gain/lose more than maybe 1-3 minutes per year. The fake ones, however, are fast/slow by 10-20 minutes per month. This is likely where they are saving money, i.e. the quartz oscillator thingy is either cheap or doesn't have calibration or whatever. The battery life is also likely far worse than most Casios, (though I have not confirmed this) which are generally excellent in this respect.

This is a watch that is regularly available on Amazon for a legit price of < $15. (Right now, it's ~$10) And there are tons of fakes out there.

1. https://en.wikipedia.org/wiki/Casio_F-91W

2. https://www.youtube.com/watch?v=lk_-_Phiklc

> 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 :)

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"!

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_p...

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.

You're missing the fact that they talked about ALREADY suspect "detainees" in the parent comment -- not the general public.

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.

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.

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.

I was responding to this:

  "Actually, they say they found time bomb detonators based on this watch."

Which, as a response to raising the false positive rate issue, is entirely missing the point.

For what it's worth: Also, you probably don't know whether this watch actually was common in Afganistan in early 2000s.

Also doesn't actually address the issue with false positive rates and conditional probability, but it is a fair point that the (clearly stated) oversimplification is, in fact, oversimplified.

But it does mean that scrutinising people with f91w watches should increase detection rates.

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.

But that may only be because the terrorists wearing those watches are worse at not being found out than the ones not wearing them.

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.

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...

So you are saying I should sell my fakes for the same price as the real ones?

Cool. Thanks for the links. I used to have a genuine Casio years ago and loved it. Ran for about five years on the same battery. Wanted to buy one again and almost bought one online.

Well, I guess the answer is that in some countries $15 is quite a lot of money, so people want to have a "casio" watch but for 1/3 of the price. That's the market these watches are made for, not the rich economies of the west.

I'm surprised the HP touchpad charger got such high regards. I admit, it seemed to do well with non Apple devices, though it would make an annoying high pitch noise when idling.