Can we still trust public opinion polls?

As the dust settles on another Super Tuesday, Americans are watching with bated breath to see how closely the final tallies align with the latest polls.

Recent years have seen public opinion polls fall from grace. In 2016, almost every national polling outfit predicted a Hillary Clinton landslide that never materialized. In 2020, while most preelection polls correctly predicted Joe Biden's victory, they also tended to overstate support for the Democratic candidate relative to then-President Donald Trump. These blunders left many Americans disillusioned with the entire polling industry.

Yet, despite these misses, polls continue to dominate headlines, saturate the airwaves and flood our social media feeds, especially as we inch closer to Election Day. With so much data swirling around, how can we tell a good poll from a bad one? USC pollsters offer some insights.

"There are a lot of moving parts in predicting elections, and these are all on top of the proper fundamentals in collecting data from the public," said Jane Junn, USC Associates Chair in Social Science at the USC Dornsife College of Letters, Arts and Sciences. "If you don't have good data to start with, no amount of tweaking will make things better."

What are signs of a strong political poll?

The strength of polls depends on when they are done, experts say.

Polls conducted during non-election periods, or when the race is many months away, require a few fundamentals: selecting an unbiased sample (making sure everyone who is eligible has an equal probability of selection), asking questions that are not slanted toward one or another side, and providing details of how the poll was done, said Junn, an expert on public opinion, political behavior, and polling methods and analysis.

But polls done during election seasons are a different beast.

"When you're thinking about calling races in a horse race, one must consider not just how people describe their vote choice in the moment, but whether or not they are likely to turn out," Junn said.

Junn suggests looking at how the pollster or outlet has defined the "turnout model," a statistical model used to estimate how many people will actually participate in an election.

"Constructing a turnout model is as much an art as it is science, with among the most influential elements in the latter being turnout among specific groups (i.e., young people, seniors or Latinx people) in prior elections, and in this case, primaries in particular. But it is important that analysts model turnout with the proper comparisons, and in this case, primaries in a presidential election year," she said.

Junn gave the example that there are more registered Democrats in California, but Republicans tend to turn out at a higher rate. This is likely why some polls show ex-Dodger and Republican candidate Steve Garvey leading U.S. Rep. Adam Schiff in the California Senate primary race.

"Do I believe this? I'm not sure, and my guess is that they've underestimated the predicted turnout among Dems and those who have declined to register with a political party preference," she said.