I cannot think of any empirical way the percentage of Muslims that are terrorists can be quantified

[LeftyMom]

2. You're approaching the situation incorrectly, I think.

First, the person you are arguing with needs to define what a terrorist is. If they're arguing that 1/4 of Muslims are terrorists they have to have a definition that involves having ideas they don't like. Their argument falls apart at this point, because any definition of terrorism that doesn't involve violence is meaningless.

Second, they need to prove their assertion. It's not your job to debunk an unevidenced assertion. In the case of something that's obviously wrong using no further evidence than the human eye, you really shouldn't waste your time.

Third, the way to get somebody who isn't a complete shithead to rethink a belief like "1/4 of Muslims are terrorists" if you think they're persuadable is to ask them questions. "What's a terrorist? How do you define that?" "Are you really saying that one in twenty human beings worldwide is a terrorist?" "If that were even remotely true wouldn't the world be much more violent than it is?" "How many Muslims do you know exactly? Which of them are terrorists? How can you tell?" etc