No consistent bias in polls for last 10 presidential races

Nate Silver / New York Times News Service /

With polls drawing increased attention in the closing weeks of the presidential race, perhaps it is no surprise that when supporters of one candidate do not like the numbers they are seeing, they tend to blame the messenger.

In 2004, Democratic websites were convinced that the polls were biased for President George W. Bush, saying they showed an implausible gain in the number of voters identifying as Republicans. But in fact, the polls were very near the actual result.

Bush defeated John Kerry by 2.5 percentage points, slightly better than the 1- or 2-point lead that he had on average in the final polls. Surveys of voters leaving polling places that year found an equal number of voters describing themselves as Democrats and Republicans, also close to what the polls had predicted.

Since President Barack Obama gained ground in the polls after the Democratic National Convention, it has been the Republicans’ turn to make the same accusations of bias. Some have said the polls are “oversampling” Democrats and producing results that are biased in Obama’s favor.

The criticisms are largely unsound, especially when couched in terms like “oversampling,” which implies pollsters are deliberately rigging their samples.

But pollsters, at least if they are following the industry’s standard guidelines, do not choose how many Democrats, Republicans or independent voters to put into their samples — any more than they choose the number of voters for Obama or Mitt Romney.

Instead, this is determined by the responses of the voters that they reach after calling random numbers from telephone directories or registered voter lists.

Data suggest that polling in presidential elections has no history of partisan bias, at least not on a consistent basis. There have been years, like 1980 and 1994, when the polls did underestimate the standing of Republicans. But there have been others, like 2000 and 2006, when they underestimated the standing of Democrats.