Why is my overall correlation higher than the subgroup correlations?

Question

I ran correlations for each grade on student test scores and the number of levels they completed on an interactive learning program. The correlations for the grades ranged from .5-.7, so why is the overall correlation (which includes all these grades) higher .85?

Answer 1

This could be Simpson's paradox: relationship appears in several different groups but disappears or reverses when these groups are combined.

You can view the illustration below:

https://commons.wikimedia.org/wiki/File:Simpsons_paradox_-_animation.gif