> Ramzi Yousef (Philippine Airlines Flight 434) and Ahmed Ressam (the 'Millennium Bomber') both had them.

This is like a statistics 101 question. Google says that Casio has sold over 100M F-91W's since 1989. Let's round down to an average of 2.5M a year, or 25M over the 10 peak years of the global war on terror (GWOT).

You named two terrorists who wore them. Let's say that's an understatement by 200X. Some quick math shows that the percent of terrorists who wore F-91W's is 400/25000000 = 0.0016% positive rate, or conversely a 99.9984% false positive rate. Not good!

Now I never actually took a statistics class, so maybe I got this question wrong. But it's obvious that using the possession of an F-91W as a sign of being a terrorist is a mistake.

Well, your argument might be valid but calculation of course has nothing remotely to do with truth. If we’re talking about the US, person has to be in the US/travel to US. That dramatically cuts down the amount from 25M. Second - the person has to be.. how do I put it, non white? Probably muslim. That also cuts it down by far. I can believe that people who fall in these two categories might have some subcultural groups. And in most of them they might just.. not wear Casio watches. It’s not like probability of wearing/not wearing Casio watches is equally likely for every person on earth. They have their target groups.