[tsearcher]

Just because the question has been asked and answered doesn't mean it can't be asked again and looked at in a different way.

I think there would be a couple of ways to find correlations for the education example you provided. First you could just compare school districts with similar household incomes and education levels, but different salaries for the teachers. Additionally, you could compare individual schools with themselves. That is, often a bond won't pass and schools have to make budget cuts. Then a year or two later the bond passes and the money is there again. So you could compare test scores before and after.