Donald Trump perpetually trumpets his lead in national polls. Bernie Sanders points to his surge from obscurity to a virtual tie in Iowa. Marco Rubio tells his supporters his showing in the Hawkeye State surpassed polling predictions.
Yes, it’s that crazy political polling season again.
Polls serve a purpose, but you have to take them, certainly at this point in the presidential campaign, with a grain of salt.
Trump outpolled rival GOP contender Ted Cruz in Iowa, but the ground game Cruz put together won the day in caucus sites. Were the polls wrong or did they just miscalculate the impact of Cruz staffers going door-to-door to nail down supporters who would brave winter cold to caucus? Turnout in elections is hard for polls to predict accurately.
Last-minute candidate surges can trick polls. They can be overstated or understated. Or missed, like Rubio’s in Iowa. Even weekly polls can be too slow to track fast-moving voter impressions.
How well candidates fare with key cohorts of voters can be missed, too. Hillary Clinton’s “upset” victory over Barack Obama in the 2008 New Hampshire primary was traced to polling samples that under-represented lower income voters who didn’t have or take the time to respond to telephone polls. The same problem can occur now if pollsters don’t include respondents only reachable on cell phones.
National polls can obscure state-level electoral leanings. Bernie Sanders may thrive in New Hampshire, which has a very liberal, white Democratic base and is next to his home state of Vermont. Hillary Clinton may have a clear advantage in South Carolina where African Americans dominate the Democratic base. Even though Cruz trailed Trump in national polls, he concentrated his efforts in Iowa on Christian evangelical voters who have a history of determining who wins the GOP vote there.
Polling techniques can have subtle influences on outcomes, which is why different polls taken at the same time with equivalent samples and sample sizes produce varying results. One of the factors in polling discrepancies is “tactical voting” or undecided voters declaring a preference they really don’t mean. When you have a lot of candidates, this factor grows in significance.
Then there is the confusion between polls and probabilities. Nate Silver of FiveThirtyEight earned a reputation – and skeptics – for basing candidate predictions on a different statistical analysis, not on the candidate's poll numbers. In a tweet following the Iowa caucuses Monday night, Silver said, “Polls in general elections = pretty good. Polls in primaries = much less accurate. Iowa caucus = especially tough.”
In a blog before the caucus, Silver said poll numbers don’t lie; they just don’t tell you the truth. “Could Marco Rubio win the Iowa caucuses despite not having led in a single poll here?” Silver wrote. "Sure. Rick Santorum did that exact thing four years ago.”
So if you are influenced by poll numbers in the early going of the presidential race, you might want to reconsider. The political polling crazy season is just beginning (again).