334 Chapter 11 Sometimes, there are competing theoretical arguments and we are agnostic about which effect is stronger. When hourly wages increase, people might choose to work more because they will be paid more, or they might work less because they need not work as many hours to pay their bills. In cases like this, where there are competing arguments, the data may tell us which effect is stronger. The important point is that we have good reasons for believing there is a relationship, even if we are unsure of the sign. We should not use a model that relates stock prices to football scores, because we have no good reason for believing that stock prices are influenced by football games. 11.2 Linear Models The most convenient model is linear. For example, we might assume that aggregate consumer spending C is a linear function of income Y, each in trillions of dollars: C = + Y (11.1) In mathematics, the symbols Y and X are used repeatedly to show relationships among variables; for example, Y = 2 + 3X. In applied fields like economics, chemistry, and physics, variables represent real things (like income, hydrogen, or velocity) and a variety of symbols are used to help us remember what the variables represent. In economics, C is typically consumer spending and Y is usually income, and this is why these symbols are used in