2016 General Election: Tracking Within-Pollster Trends

Make no mistake, when trying to gain a signal from the onslaught of horserace polls that arrives this time of year, aggregation of surveys as a whole is the best way to go. Averaging the most recent polls or taking their median should give you the clearest picture. However, that’s not to say breaking up these numbers by different survey characteristics won’t return interesting results or speak to important trends (e.g. could a social desirability bias be coming through when you compare live interview vs. online polls?).

I’ve checked polling developments by survey mode, differences by party support for each candidate, and briefly keened in on one particular pollster in the past. Here I’ll expand on that last point: trying to gauge differences by pollster. Overall aggregation is always most important, but often times within-pollster trends can add a new layer of explanation for each candidate’s position in the general election race.

Using data from HuffPost Pollster, I track below the results from five polling outlets that have most often conducted head-to-head matchup polls between Hillary Clinton and Donald Trump. I restrict polls to those that test only these two candidates (i.e. for survey questions in which third party candidate names are not listed), and where the population comprises both registered and likely voters.

Though noisy early on, here’s how the Clinton’s margin over Trump–subtracting Clinton’s percentage support from Trump’s–looks like over the last year.

Data source: HuffPost Pollster.

Only ABC/Post, CNN, and Morning Consult have been asking this trial heat question for nearly the entire year. A mid-May ABC/Post poll that showed Trump ahead affects that pollster’s trend a lot, but for the most part all of the pollsters show Clinton consistently ahead. They don’t all follow similar paths, especially when all of them are consistently sending out polls in the last few months.

On that note, it’s worth restricting the time interval of focus to the start of June onward–the period following Trump clinched the Republican nomination. The trends should be more meaningful during this time, as it captures the process of party reunification/usage of partisanship heuristics (or lack thereof) and how the electorate reacts more broadly to just two competitive candidates in the race. Below, I graph the poll results from these same five pollsters, only starting after Trump secured the nomination (note: I use line graphs rather than the smoothing lines above because of the fewer data points).

Data source: HuffPost Pollster.

For nearly all of the different frequent pollsters, the Clinton margin follows the schedule of the party conventions and their ensuing effects: Trump closes the gap in mid to late July after the Republican convention wraps up–see the sharpest drops for the CNN, Morning Consult, and NBC/SurveyMonkey polls in the graph–but Clinton responds by restoring her lead following her party’s convention. This latter effect has likely proved most durable, as even weeks after the DNC, the trends for all pollsters show Clinton’s margin rising (that likely has as much to do with Trump’s various moments of implosion as well). It seems Ipsos/Reuters was least receptive to this convention-related development, as that firm shows around a five percentage point decline for Clinton starting in early June.

Right up until the last few weeks, the pollster most favorable to Trump has been the Morning Consult, which conducts its surveys online (even at this point, it’s second most). From the five pollsters examined here, that firm held the nadir for Clinton’s margin in the last few months, as well as most often had Clinton with the lowest lead. That’s not the first time Trump has succeeded specifically in Morning Consult polls. If you followed pollster discrepancies during the Republican primary at all, you would recall Morning Consult often yielded the most bullish results for Trump, representing the first poll that consistently had Trump’s primary support at the national level in the 40s percentage-wise.

Ipsos/Reuters and NBC/SurveyMonkey also conduct polls online but haven’t shown Trump as strong during the general election, so it’s hard to ascribe this result for Morning Consult to an effect of online surveys. Maybe it has something to do with question ordering and resulting priming effects for survey-takers. For example, in Morning Consult’s latest poll, it asks about several favorability/approval ratings before getting to presidential candidate matchups. I also thought that if the firm employed a panel, it might have something to do with the characteristics of this group that kept coming back to take each survey–but Morning Consult does not allow a single respondent to take a survey more than once a year. In general, according to its methodology, it seems the firm doesn’t deviate much from the practices of other pollsters (in terms of how it weights, its use of CPS data, etc.). It’s thus hard to pinpoint for certain why Morning Consult has strayed from the polling pack–not in magnitude but in consistency.

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Clinton vs. Trump: Differences by Survey Mode

Back in early January in the run-up to the first primaries in 2016, the debate over what different type of surveys signified hit a high pitch. Highlighting the divide between traditional live interview telephone polling and anonymous online survey-taking, the advantage held by Donald Trump in national and state polls was called into question. Outperforming in online formats led many–including myself–to believe that his actual level of support was getting spuriously inflated. After Iowa, that notion became much less convincing in light of Trump’s successes.

Now, this niche topic concerning survey mode has returned, but has gained attention from the broader public:

As a response to Trump’s precipitous fall in both state and national polls, his support network has responded by pointing to a supposed “hidden vote” lurking in the electorate. The claim directly ties in what different types of surveys are telling us: a Trump advisor says that this unaccounted advantage of Trump’s lies in online polls, where she claims he does better than in live interview polls, and that this is a better indication of the state of the race since online polling better approximates the act of voting (done anonymously).

To start, it’s important to note this alludes to a crucial dynamic that plagues surveys in which the interviewee directly interacts with a live person interviewer: social desirability bias. People may be less willing to reveal certain opinions that are less socially acceptable, and thus undermine the objective of opinion research of accurately capturing public attitudes. During several points during this election cycle, this issue’s relation to Trump’s polling numbers has received insightful and legitimate consideration.

But is the current claim coming out of Trump’s camp valid? To try to find any difference by survey type in Trump support–and see if live phone interviews substantially undersold Trump’s position relative to online surveys–I examined the margin of Hillary Clinton’s lead in live phone and internet polls over the last year.

Source: HuffPost Pollster.

Note: This data is from polls that only test head to head matchups between Clinton and Trump, which exclude mention of third party candidate names. 

The dotted line in the above graph approximates Clinton’s margin in live phone surveys, and the solid line does so for internet polls. While Clinton consistently leads Trump by a larger margin in phone interviews than in online ones throughout the time frame, the difference–maybe around a percentage point–is far from anything really substantive. That small gap in survey modes generally stays constant going back to general election trial heats conducted in late 2015 as well. Simply put, in the context of a Trump campaign in serious polling peril, pointing to potential survey mode effects may temporarily divert attention, but–at this point–seems very unlikely to have a discernible effect come November.

Update on Trends in Intra-Party Support

Having now emerged from the most important juncture–the party conventions–in the 2016 presidential election since the end of the primary season, it’s worth taking stock of the degree of support Hillary Clinton and Donald Trump are receiving from within their respective parties. At several times earlier this year, party coalescence seemed unlikely, or at least to the extent seen in past electoral cycles. That notion gained support from earlier polls showing subpar Democratic support for Clinton, due to holdouts from Bernie Sanders supporters, and below average Republican numbers for Trump, stemming from reticence of party elites and high socioeconomic (Republican) voters to his campaign.

For the most part, intra-party support has come close to familiar levels. Party loyalty, as expected, remains one of the strongest forces in American politics, and the two candidates have seen their overall polling numbers rise as a result. But as seen below in this brief overview of intra-party developments in the last two months in the graphs below, the trends for Clinton and Trump have been a bit uneven.

Source: HuffPost Pollster.

Source: HuffPost Pollster.

Note: The x-axes in the above graphs start with a poll that began on 6/1, and end with another poll that concluded on 8/5. The polling data used is only from Clinton-Trump head to head matchups that do not include third party candidates by name. 

Clinton sees her support from Democrats more or less consistently rise over this two-month time frame, with a larger boost coming around the time of the Democratic Convention. Trump also gathers more support from Republicans following the party’s convention, but that trend towards party unity halts thereafter, likely a repercussion of the various controversies in which Trump has recently been embroiled.

If Trump hopes to rebound and make the election competitive once again, he’s best off start with pulling in members of his own party. Clinton, on the other hand, has managed to do that much more effectively, and as a result, stands in much better shape–in her polling numbers for all voters–in the post-convention landscape.