Each of these organizations differ in their approaches. Real Clear Politics does a more straightforward averaging of the most recent polls. TPM’s PollTracker is an aggregation involving regression analysis that uses the most recent polls to project a trajectory for the race. FiveThirtyEight and HuffPost Pollster use polls, adjusting them for house effects–the degree to which a survey house’s polls lean consistently in one direction or another. FiveThirtyEight also uses non-survey data to project the election results.

All four of these outlets underestimated Obama’s margin of victory. Both Real Clear Politics and PollTracker had Obama ahead by only 0.7 percentage points in their final measurements. HuffPost Pollster had Obama leading by 1.5 points, while FiveThirtyEight was closest, showing Obama 2.5 points ahead of Romney in the last estimate. The aggregators that came closest to Obama’s overall winning margin were the ones that attempted to account for pollsters’ house effects.

“The polls, on balance, understated President Obama’s support,” said John McIntryre, cofounder of Real Clear Politics. “Our product is only as good as the quality and the quantity of the polls that we use.”

These sorts of house effects were why HuffPost Pollster moved to a model that attempted to control for them, but their average still underestimated Obama’s margin of victory by a sizable magnitude. “One of the main reasons why we moved to using a more complex model that controlled for house effects was precisely to prevent that phenomenon from happening,” Blumenthal said. “Our goal is to minimize that to next to zero.”