The criteria is this, featured in our What topics can I ask about here? help article (the only one we get to customise):
Question about a general real-world topic such as history, geography or economics, whilst relevant to RPGs, may be off topic if they are not RPG-centric (or better belong on another Stack Exchange site, such as History). A good rule of thumb is to ask yourself …
Would an RPG expert give me a better/different/more specific answer to this question than a Historian, Geographer, etc?
If yes, then feel free to ask it here.
That's our rule of thumb for real-world topics. We arrived at a decision to use this back in 2012 when we couldn't iron out where to draw the line on campaign research questions, but it is not limited to just campaign research.
For example, Is 3d6 the same as 1d18? is another recent question that instead of being closed, received one of the highest vote counts in recent months.
It received one of the highest vote counts because it was in the hot questions list, and that's what happens to some questions that are in the hot questions list. People came, learned, registered, and voted. You might notice that one of the answerers, Eric Lippert, renowned for such things as his work on the C# programming language, has never used our site prior.
Why's it open though? I have no idea, and I voted to close it when it arrived.
It doesn't pass our rule of thumb. It involves dice, and dice probabilities are relevant to us, but answering that question requires no RPG expertise, and RPG experts have not given a better, different, or more specific answer than something a mathematician would.