From the Pew Research Center: Key things to know about election polling in the United States

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A robust public polling industry is a marker of a free society. It’s a testament to the ability of organizations outside the government to gather and publish information about the well-being of the public and citizens’ views on major issues. In nations without robust polling, the head of government can simply decree citizens’ wants and needs instead.

After the 2016 presidential election, some observers understandably questioned whether polling in the United States is still up to the task of producing accurate information. Errors in 2016 laid bare some real limitations of polling, even as clear-eyed reviews of national polls in both 2016 and 2018 found that polls still perform well when done carefully.

One way to help avoid a repeat of the skepticism about surveys that followed the last presidential election is to narrow the gap between perception and reality when it comes to how polling works. People have many notions about polling – often based on an introductory statistics class, but sometimes even less – that are frequently false. The real environment in which polls are conducted bears little resemblance to the idealized settings presented in textbooks.

With that in mind, here are some key points the public should know about polling heading into this year’s presidential election.

- Different polling organizations conduct their surveys in quite different ways.

- The barriers to entry in the polling field have disappeared.

- A poll may label itself “nationally representative,” but that’s not a guarantee that its methodology is solid.

- The real margin of error is often about double the one reported.
There is evidence that when the public is told that a candidate is extremely likely to win, some people may be less likely to vote.

- Estimates of the public’s views of candidates and major policies are generally trustworthy, but estimates of who will win the “horse race” are less so.

- All good polling relies on statistical adjustment called “weighting” to make sure that samples align with the broader population on key characteristics.

- Failing to adjust for survey respondents’ education level is a disqualifying shortfall in present-day battleground and national polls.

- Transparency in how a poll was conducted is associated with better accuracy.

- The problems with state polls in 2016 do not mean that polling overall is broken.

- Evidence for “shy Trump” voters who don’t tell pollsters their true intentions is much thinner than some people think.

- A systematic miss in election polls is more likely than people think.

- National polls are better at giving Americans equal voice than predicting the Electoral College.