3. We know U.S. political candidates spend an awful lot of time raising money. And we know they use the money to inflict mind-numbing ads on us. Do we know if the money and the ads it buys actually work? That is, does campaign spending increase vote share? Jacobson (1978), Erikson and Palfrey (2000), and others have grappled at length with this issue. Consider the following model: Vote share;= Po + B Campaign spending; + where Vote share; is the vote share of a candidate in state i and Campaign spending; is the spending by candidate i. (a) What does o mean? What does mean? (b) What is in the error term? (c) What are the conditions for the independent variable X to be endogenous? (d) Is the independent variable likely to be endogenous? Why or why not? (e) Explain how endogeneity could lead to incorrect inferences.