Students who study more do tend to get higher grade, but other factors also influence a student’s grade. Previous preparation is an important factor, for instance, as are talent, attention from teachers, even eating a good breakfast. A scatter plot like Figure A-2 does not attempt to isolate the effect that study has on grades from the effects of other variables. Often, however economists prefer looking at how one variable affects another, holding everything else constant. To see how this is done, let’s consider one of the most important graph in economics: the demand curie. The demand curve traces out the effect of a good’s price on the quantity of the good consumers want to buy. Before showing a demand curve. However, consider Table a-1 which shows how the number of novels that Emma buys depends on her income and on the price of novels. When novels are cheap, Emma buys them in large quantities. As they become more expensive she borrows books from the library instead...