Public-opinion polls have long been a part of the U.S. political scene. They help candidates understand what’s on voters’ minds, and they give everyone a sense of who’s ahead — or seems to be — weeks, months and even years before the next election.

But the polls aren’t always right. A leading 1936 poll wrongly predicted the defeat of incumbent President Franklin D. Roosevelt, and Thomas E. Dewey went to bed on election night in 1948 believing he’d been elected president, only to discover in the morning that he had not.

The scientific basis of polling is sound. Ask a sufficiently large and representative sample of the public a number of fairly worded questions and you can determine with a high degree of accuracy the views of the general population. Polling pioneers like Elmo Roper, George Gallup and Louis Harris predicted election results with a high degree of accuracy. Modern presidential candidates all have sophisticated polling operations of their own.

For many years, political polls have offered insight on questions like:

  • “Is the country going in the right direction?”
  • “How much will each of the following issues determine your vote in the next election?” And, of course …
  • “If the election were held today, would you vote for Candidate A or Candidate B?”
University students in New York state work as phone pollsters in 2005. (© AP Images)

But the 2016 campaign has proved challenging for pollsters. One reason is Americans’ growing reliance on mobile phones rather than traditional landlines. Federal law permits machine dialing (“robocalls”) of landline numbers but forbids it to mobile phones. Calls to those numbers must be placed by humans, and that’s slower, less efficient and more expensive.

Some pollsters cut costs by sampling a smaller slice of the population, or by oversampling landlines. Either strategy can make the results less accurate.

And some observers suggest that citizens don’t always respond truthfully when polled, not wanting to give what they perceive as unpopular responses to poll questions.

Even so, polls remain a prominent feature in news coverage of the 2016 race. They’re a good way to stay informed, but remember: The only poll that counts is the one on Election Day.

Graphic reading "Elections 2016" (State Dept./J. Maruszewski)