help to think of regression analysis as performing the following task: converting each of those twenty thousand schoolchildren into a sort of circuit board with an identical number of switches. Each switch represents a single category of the childs data: his first-grade math score, his third-grade math score, his first-grade reading score, his third-grade reading score, his mothers education level, his fathers income, the number of books in his home, the relative affluence of his neighborhood, and so on. Now a researcher is able to tease some insights from this very com- plicated set of data. He can line up all the children who share many characteristics-all the circuit boards that have their switches flipped the same direction-and then pinpoint the single characteristic they dont share. This is how he isolates the true impact of that single switch on the sprawling circuit board. This is how the effect of that switch- and, eventually, of every switch-becomes manifest. Lets say that we want to ask the ECLS data a fundamental ques-     tion about parenting and education: does having a lot of books in your home lead your child to do well in school? Regression analysis cant quite answer that question, but it can answer a subtly different one: does a child with a lot of books in his home tend to do better than a child with no books? The difference between the first and sec- ond questions is the difference between causality (question 1) and correlation (question 2). A regression analysis can demonstrate corre- lation, but it doesnt prove cause. After all, there are several ways in which two variables can be correlated. X can cause Y; Y can cause X; or it may be that some other factor is causing both X and Y. A regression alone cant tell you whether it snows because its cold, whether its cold because it snows, or if the two just happen to go together. The ECLS data do show, for instance, that a child with a lot of books in his home tends to test higher than a child with no books. So those factors are correlated, and thats nice to know. But higher test