A Case for Political Momentum

Whenever I teach statistics, we usually spend a day on the "hot hands" phenomenon--the perception that, during a game, a sports player is "on fire," or "on a roll," in other words, has had a really great game up to that point, and thus, will likely continue to have similar success the rest of the game. However, the data does not support this belief (although newer research has problematized this conclusion). In fact, it has been given it's own name--the "hot hands fallacy." By way of analogy, some argue that there is no such thing as "political momentum"--that any "streak" that one sees, such as Bernie Sanders' seven wins from March 22- April 9, is simply that states already amenable to Sanders just happen to fall on consecutive dates.

While I accept the data about the hot hands fallacy, a correlational analysis of the current presidential primary race seems to indicate that a pattern has evolved on the Republican side for Trump and Kasich (not so much for Cruz). For this simple analysis, I took the percent wins for five candidates: Sanders and Clinton on the Democratic side, and Trump, Cruz and Kasich on the Republican side. In addition to the percent wins for each candidate, I also did a "margin of win" calculation for Sanders-Clinton, and Trump-Cruz. Then I ordered all of the results by date for both parties, beginning with the Iowa caucuses on February 1, to the April 26 primaries. The Democrats have had 40 such votes, while the Republicans have had 38. Then I performed a correlation on Excel, using Feb 1 as "Day 1" and April 26 as "Day 85," against each of the seven outcome measures--the percent wins for the five, or the margin of win for the two specific comparisons.

On the Democratic side, all three measures (Sanders-Clinton, Sanders % win, Clinton % win) had no time-based correlation, with an r-value of less than 0.07 in all cases. Typically we don't care about r-values less than 0.20, and we often only get excited with r-values greater than 0.50 (depending on what we're measuring, of course). Similarly, on the Republican side, the r-value for Cruz's wins over time is -0.16, implying that he is losing votes as the primaries unfold, although not by much--a statistically insignificant decrease.

However, Trump, Kasich, and the Trump-Cruz win margin shows quite a bit of increase over time. The latter shows the lowest correlation, r=0.41, implying that as time passes, Trump's win margins over Cruz are increasing. That might not make sense given that Cruz's win percents are remaining relatively the same. However, the best explanation for this is that as 14 of the 17 original GOP candidates dropped out of the race, they tended not to go for Cruz, but were split between Trump and Kasich, while Cruz got very few of those votes.

Trump has shown the greatest increase over time, r=0.64, a surprisingly strong relationship. Kasich's wins have also increased, although not by as much, with r=0.52 as measured over time. A graph of the votes makes this finding more clear. As you can see, Trump's win percents remained relatively steady from Feb 1-March 22. However, in April there were seven states who voted, and his success has tremendously improved. Cruz seemed to show a steady improvement from the beginning, through the March votes--but then sank back down to his earliest vote totals with the April votes. Finally, Kasich has been gaining percent wins steadily, and finally surpassing Cruz with the April votes. Is this "momentum"? It could certainly just be the coincidence of states amenable to Trump and Kasich. After all, the April 26 election was just states near New York. Not only is that Trump's home territory, but Cruz has said unkind things about New York, which likely cost him dearly in those states. Regardless, the correlations show a strong relationship for both Trump and Kasich, so the "momentum" claim might actually pan out for those two.