It's not quite as simple as "if you have one of these you're a terrorist" of course; it's one indicator based on long-standing patterns, but never used as sole proof.

AQ training/bomb-making manuals from the 1990s–2000s recommend cheap, reliable digital watches like this as timers for IEDs as they're extremely inexpensive and widely available worldwide, durable, water resistant, and easy to modify as delay timers.

Often these guys would get picked up with two or more watches of the same model, for example. We recovered them in bomb-making facilities and on fighters throughout the GWOT. Ramzi Yousef (Philippine Airlines Flight 434) and Ahmed Ressam (the 'Millennium Bomber') both had them.

How many watches did Khalid El-Masri have?

https://en.wikipedia.org/wiki/Khalid_El-Masri

> Khaled El-Masri is a German, Lebanese and French citizen who was mistakenly abducted by the Macedonian police in 2003, and handed over to the U.S. Central Intelligence Agency (CIA).

> While in CIA custody, he was flown to Afghanistan, where he was held at a black site and routinely interrogated, beaten, strip-searched, raped, and subjected to other forms of torture.

> After El-Masri held hunger strikes, and was detained for four months in the "Salt Pit", the CIA admitted his arrest was a mistake and released him.

Trick question! It was actually that his name is one letter away from someone else's name, and some manager in the CIA had "a hunch":

> At the end of 2003, El-Masri travelled from his home in Ulm to go on a short vacation in Skopje. He was detained by Macedonian border officials on 31 December 2003, because his name was identical (except for variations in Roman transliteration) to that of Khalid al-Masri, who was being sought as an alleged mentor to the al-Qaeda Hamburg cell, and because of suspicion that El-Masri's German passport was a forgery.

> CIA agents discussed whether to remove El-Masri from Macedonia through an extraordinary rendition. The decision to do so was made by the head of the al Qaeda division of the CIA's Counter-Terrorism Center, Alfreda Frances Bikowsky, on the basis of "a hunch" that El-Masri was involved in terrorism; his name was similar to suspected terrorist Khalid al-Masri.

> When the Macedonian officials released El-Masri on 23 January 2004, American security officers immediately kidnapped him. El-Masri later described them as members of a "black snatch team." They beat him and sedated him for transport using a rectal suppository. "The CIA stripped, hooded, shackled and sodomized el-Masri with a suppository—in CIA parlance, subjected him to "capture shock"—as Macedonian officials stood by."

> 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.

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