QUESTION 1 - INCUMBENCY ADVANTAGE

One of the most remarkable features of electoral politics in the United States is the high degree of electoral success of incumbent candidates. For example, for the last five decades, conditional on running for re-election, incumbent candidates have won elections to the House of Representatives about 90 percent of the time. This phenomenon has prompted much empirical research aimed to estimate the "incumbency advantage," that is, the causal effect of incumbency on the vote share obtained by a candidate.

Suppose that you have data on every two-candidate race (with both a Democrat and Republican running) to the House of Representatives for some period of time. You want to estimate the "incumbency advantage" for Democratic candidates.

For each of the following estimators, describe possible sources of selection biases. Explain which direction you would expect the results to be biased and why.

(a) "Vote Share Difference"the difference in average vote shares between Democratic incumbents and Democratic non-incumbents.

(b) "Sophomore Surge"among those Democratic candidates who were elected to office and who run for a second term immediately after the first term, the difference in average vote shares between the two elections. (That is, the average vote share gains for freshmen winners who run again in the following election.)

(c) "Retirement Slump"in districts where the Democratic candidates retire, the difference in average vote shares obtained by the retiring Democratic incumbents and the incoming Democratic candidates (who clearly are not incumbents) in the current election.