Understanding Simpson's Paradox

When you look for overall trends, you often poke around the data in aggregate, but when you zoom out too far, you could miss details or within-category variation. Sometimes when you zoom in, you see a completely opposite trend of what you saw overall. This is known as Simpson's Paradox.

http://flowingdata.com/2013/09/19/a-visual-explanation-of-simpsons-paradox/