With the release of Hillary Clinton's campaign memoir last week, political observers have turned their attention back to the 2016 presidential race.
And that, inevitably, leads to James Comey. A central debate in the campaign post-mortems is whether Comey, then the FBI director, tipped the election in Donald Trump's favor with the Oct. 28 release of a letter announcing the review of possible new evidence in the Clinton email investigation. Some journalists, political analysts and Clinton herself contend that the letter's release cost her the presidency. "Absent that," Clinton told NBC last week, "I believe the evidence shows I would have won."
In a recent paper, however, we cast doubt on the claim. Using an analysis of polling data from the last four months of the campaign, we find no conclusive evidence that the Comey letter led to a decisive shift in voter support. Instead, changes in support for the candidates resulted largely from other factors.
We arrived at this conclusion through an analysis of Clinton and Trump's standings in national polls from July 1 through the day before the November election. We use data from Pollster's 2016 General Election poll chart. Specifically, our measure is Clinton's share of the two candidates' support.
We measured the effect of the Comey letter by including a variable to examine changes in the polls on Oct. 28 - the day the letter was released - and in the days afterward. If the letter hurt Clinton, we would have found a statistically significant decline in her lead over Trump, above and beyond what the other variables would tell us. This is a standard way of measuring whether a campaign event has a durable effect on the polls.
Our model shows that the Comey letter led to a small drop in Clinton's lead, but that shift is not statistically significant. In other words, there is no clear evidence that the letter cost Clinton votes. When we specified our model in different ways - for example, looking at the impact for just the three days after its release - we again did not find evidence that the letter moved the polls.
It is worth noting that we also found no indication that the release of the Access Hollywood tapes, in which Trump bragged about his sexual exploits with women, had an effect. What mattered most was President Barack Obama's approval rating - when it went up, so did Clinton's support - and the Democratic National Convention and the first presidential debate. Both of those increased Clinton's lead in the polls.
There are, of course, limitations to the analysis. We have relatively few observations (130 days), which means that there is likely to be statistical uncertainty associated with any variable, including the Comey letter's release. That will necessarily make it harder to find "significant" effects.
In addition, our measures of media coverage don't account for the tone of coverage toward Trump and Clinton. That means we can't determine whether the Comey letter might have had an indirect effect on voter support by making the news about Clinton more negative.
But an analysis by the Shorenstein Center at Harvard University found that the tone of media coverage was similar for Trump and Clinton during the last three months of the campaign. As a result, the measure of the volume of coverage in our model may be just as useful as a direct measure of tone.
Finally, our data set of national polls didn't allow us to determine whether small effects from the Comey announcement (or other campaign events) might have mattered in states such as Pennsylvania, Wisconsin and Michigan, where Trump's margin of victory was very small. That is worth additional scrutiny.
But based on the evidence at hand, we see no reason to conclude that the Comey letter produced shifts in Clinton's support that were sizable, persistent or consequential.
Panagopoulos is a professor of political science at Northeastern University. Weinschenk is an associate professor of political science at the University of Wisconsin at Green Bay.
