I used to believe that the South Park proof of at least 25% of all people being idiots was the best theory yet devised, since it's hard to use objective census data to identify who is or is not an idiot, and scientific polls would rely on self-identifying idiots for their data, which obviously would result in under reporting.
The South Park theory is less scientific and more simple, but I've yet to find anything problematic about it. You and three peers are having a discussion. Based on that discussion, you can quickly confirm that among the four of you, at least one is an idiot. (Note: if you test this theory multiple times and fail to find any idiots, abandon any further investigation of this theory and make sure you're all caught up on The Walking Dead)
Kyle: Anybody who thinks 9/11 was a conspiracy is a retard!
Cartman: Oh really? Well did you know that over one-fourth of people in America think that 9/11 was a conspiracy? Are you saying that one-fourth of Americans are retards?
Kyle: Yes. I'm saying one-fourth of Americans are retards.
Stan: Yeah, at least one-fourth.
Kyle: Let's take a test sample: There's four of us, you're a retard, that's one-fourth.