Proof that Krugman is lying scum

In a recent column Paul Krugman argued that since Wisconsin has unionized schools (true), Texas has non-unionized schools (true), and educational achievement scores are higher in Wisconsin than in Texas (true), it must be that teacher unions are good for schools.

The conclusion is false even though each supporting fact is true. White Anglos in Texas do better than white Anglos in Wisconsin. Hispanics in Texas do better than Hispanics in Wisconsin. Blacks in Texas do better than blacks in Wisconsin.

How can this be? Simple. Whites on average do better than Hispanics and blacks, and Texas has a much higher proportion of blacks and Hispanics.

This is the well-known Simpson paradox, which is discussed in elementary statistics textbooks. One could argue that Krugman is just an idiot of monumental proportions, but nobody gets a PhD in economics or wins the Nobel Prize in economics without understanding Simpson's paradox.

The conclusion is that Krugman understood all this and decided to use it in his column anyway because he's the lying partisan scum we all suspected him to be.