[Zarel]

Pirates being correlated with global warming isn't a coincidence; pirates decreasing and global warming are both (indirectly) caused by the Industrial Revolution.

It's true that you can never be completely sure that any correlation isn't a coincidence, just like you can't be sure that the sun will rise tomorrow. You can, however, be sure beyond a reasonable doubt with enough of a sample size and a strong enough correlation.

Out of curiosity, do you personally believe it's likely (say, >5% chance) that it's a coincidence that girls have better grades than boys?