Several of you have asked whether the candidate who wins the “bounce war” – who gets the biggest bounce in approval coming out of their respective convention – is more likely to win the general election. In response, I went back and looked at the bounce winner dating back to 1960. In 7 of 11 elections, the winner of the bounce went on to win the general election (I did not include Gore’s victory in the 2000 “bounce” war, since he won the popular vote, but lost the Electoral College vote). So, since McCain won the “bounce” this year, does this indicate that he is more likely to win the general election? I discussed this question with my colleague, Professor Amy Yuen, and she ran some simple statistical tests to try to find an answer. Without going too deeply into the statistics, what we want to do is compare our sample’s outcome – 7 out of 11 bounce winners go on to win the general election – with what we think is the “real” underlying relationship between the bounce and the election outcome. We might think that the winner of the bounce should always win the election. In this case, we ask whether our sample outcome is, statistically speaking, different from the “real” relationship in which the bounce winner actually goes on to win the general election every time.
Another way to think about it is to assume that there is no relationship between winning the bounce and winning the general election. That is, a candidate’s odds of winning the election after winning the bounce are really no better than 50/50. So here the comparison is between winning 7 of 11 times versus winning 5.5 out of 11.
In short, we are asking the same question, but testing it in two different ways. So, what does our statistical testing tell us? Based on past history, can we infer that McCain is more likely to win the general election because his post-convention bounce was bigger than Obama’s? Amy was kind enough to run the statistical tests (all tests using the STATA 10 statistical program) and found that, in fact, one cannot be confident that there is any difference (within a commonly accepted level of statistical significance) between simply flipping a coin and the bounce winner going on to win 7 out of 11 elections. On the other hand, we can reject the idea that winning 7 of 11 times is, statistically speaking, equivalent to the bounce winner going on to win every election. In short, both results indicate that we can say that McCain’s winning the bounce does not tell us that he will win the general election; statistically, we find no relationship between the two events. (By the way, if you are interested in learning more about this test and about political science research methods more generally, I recommend taking my colleague Bert Johnson’s course Frontiers in Political Science Research this fall. He takes you through a variety of research issues, many of which we will touch on in the coming weeks in this blog. I hope to have both Amy and Bert guest blog on issues touching on their specialties as time permits.)
Having said this, there is tantalizing evidence based on the post-convention survey data, that something is happening among likely voters in this race as a consequence of the two conventions. I’ll address this issue in my next blog.