With polls showing an increasingly narrow race between Hillary Clinton and Donald Trump, it’s possible that the first presidential debate could have outsized significance, rallying one candidate’s flagging supporters or swaying the unusually high percentage of undecided voters.
It’s also possible that, like many past debates, it could change very little.
Either way, there will be no good way of knowing for sure on Monday night.
It’s not that there won’t be plenty of data available more immediately ― whether that means panels of voters convened on cable news, reader polls splashed across the internet, or numbers gleaned from social media.
But as we wrote during the primary debates last year, those measures simply aren’t very reliable at actually capturing public sentiment:
A focus group led by GOP pollster Frank Luntz after the first Republican debate in August  suggested that voters had soured on Donald Trump — a prediction not borne out in the business mogul’s poll numbers, which continued to rise sharply.
Initial reactions to the first Democratic debate also proved to be dissimilar from the results of later polling on the race. Another Luntz focus group named Sen. Bernie Sanders (I-Vt.) the clear winner, and suggested many of Clinton’s supporters had deserted her. Other focus groups and instant online polls told a similar story, as did a surge of web searches and Facebook follows for Sanders. Days later, though, scientific surveys showed a more quotidian outcome: While both candidates made a good impression, the debate, if anything, strengthened Clinton’s lead.
Why? For one thing, neither approach is very good at representing the American electorate as a whole. Like attempts to tally up crowd sizes or yard signs, they focus on a small, not particularly representative, slice of the public:
Online “instant polls,” which don’t attempt to weight their responses and may allow people to vote multiple times, often end up rewarding enthusiasm over numbers, reflecting the views of a small group of supporters rather than the nation’s debate-watchers as a whole.
“The results rely on a self-selecting group of respondents with no regard to political affiliation, age, country, or even whether the person doing the responding actually watched the debate,” Slate’s Josh Voorhees wrote...
...Focus groups, a similarly imperfect snapshot of the American electorate, are also often dominated by the loudest voices.
“While these discussions make for far more compelling television than dry survey statistics, they have important limitations,” HuffPost’s Mark Blumenthal wrote in a 2008 column. “Every group is a small, non-random sample, and it is hard to know the degree to which the views of participants may be influenced by the atmospherics of the telecast, the probes of the moderator or the opinions expressed by others in the group.”
So, finding out whether viewers think one candidate performed objectively better is a task in itself. Scientifically conducted polls of debate watchers may be able to shed a little more light on what viewers thought of the event.
But even if there’s a clear “winner” by that metric, it still won’t provide much insight into what conclusions the electorate as a whole will draw, let alone how, if at all, those opinions will end up changing the dynamic of the presidential race.
The next wave of national and state polling, which will likely start rolling out later this week, will offer more information still. As political scientist John Sides notes, though, they may exaggerate any shifts if disappointed supporters of the “losing” candidate end up feeling less inclined to answer polls, and any gains could easily be wiped out after future debates.
And then there’s another factor: the tenor of media reaction in the coming days.
Waiting to quantify the debate’s effects until long after the closing statements means that the conclusions will necessarily be affected by the way the debate is covered in the media. That, however, is more of a feature than a bug ― studies have shown the “spin war” following the debate can play as big of a role in shaping voters’ opinions as the event itself.