Gallup Performs Poorly When Out of Consensus
Usually, when a poll is an outlier relative to the consensus, its results turn out badly.
You do not need to look any further than GallupÂ’s track record over the past two election cycles to find a demonstration of this.
In 2008, the Gallup poll put Mr. Obama 11 points ahead of John McCain on the eve of that NovemberÂ’s election.
That was tied for Mr. ObamaÂ’s largest projected margin of victory among any of the 15 or so national polls that were released just in advance of the election. The average of polls put Mr. Obama up by about seven points.
The average did a good job; Mr. Obama won the popular vote by seven points.
The Gallup poll had a four-point miss, however.
In 2010, Gallup put Republicans ahead by 15 points on the national Congressional ballot, higher than other polling firms, which put Republicans an average of eight or nine points ahead instead.
In fact, Republicans won the popular vote for the United States House by about seven percentage points — fairly close to the average of polls,
but representing another big miss for Gallup.
Gallup vs. the World - NYTimes.com