In the midst of an otherwise very interesting article about Batting Average on Balls In Play:
"Our regression model yields an R-squared value of .348, and all non-vector explanatory variables are significant at the 1 percent level. This suggests that the factors included are all highly significant, and jointly explain roughly 35 percent of the variance in a hitter’s BABIP. As an additional test of accuracy, we find a robust 59 percent correlation between actual and predicted BABIP for all players in our sample."
Imagine one of your college math lectures. After the professor fills the chalkboard with algebra, he concludes with a flourish, "so x^2 = 4, as we anticipated. But, as an additional test of accuracy, we see that x = +/-2." That's basically how I read this excerpt.
Still, it's a good article, and if you're into hardcore baseball math, you should read the whole thing.