A couple of years ago my workplace was running one of those guess-the-number-of-jellybeans-in-the-jar competitions. I don't even like jellybeans all that much, but nonetheless, I held aloft my nonmagic calculator and said "by the power of Galton!" Taking the mean of all the previous guesses, I put that down as my answer. I was out by one bean, and won the jar. I don't think my colleagues have ever been so interested in statistics as they were that afternoon, and I doubt they ever will be again.
I'm going to admit something a bit silly and embarrassing now: that made me feel like a wizard. Not because of the scope of what I'd done, since it was an utterly trivial piece of arithmetic, but because of the reaction it got. I had drawn on arcane lore unknown to my colleagues, and used it to exercise power over the world.
Personally, I think something like solid state semiconductor technology is about as impressive a real-world miracle as one could ever want by way of demonstrating the whole Science Works/Rationality Is Systematised Winning/Maths Has Manifold Real-World Applications thing, but for most people it will never have the impact of intentionally winning a jar full of jellybeans.
So I ask you, LW-readership: what other impressive nonmagical powers do we have, that we can casually demonstrate to everyday people in everyday circumstances?
Great, I feel like we're making good progress. (wisdom of the crowd..)
Yes, for example from here, if the standard deviation of the individual guesses is s, the standard deviation of the average of N guesses will be s / sqrt(N).
... And this represents the typical error of seven_and_sixes strategy, within one standard deviation.
Now -- to see if the strategy typically wins -- we just need the number for: given N guesses, what is the expected minimum error of the N guesses? (That is, the average minimum of the set of differences between each guess and the mean?)
I would guess that this is proportional 1/N, whereas the average method gives 1/sqrt(N), so the probability that you win is O(1/sqrt(N)). In reality it would be worse, since, if you were not the last to go, you would only have the average of M guesses, with M < N. However, the average person has a probability of winning of 1/(N+1) so your probability of winning is sqrt(N) times better than the average person's, unless you can do even better with some other skill that you have. This analysis is complicated by exceptionally bad guessers and other average-takers, but not significantly. I accept your downvote because sixes was still lucky to win and did not acknowledge this.