Gallup is well-known for its presidential job approval poll, which goes back to the Truman presidency. Gallup also conducts polls that forecast the winner of the presidential election every four years.
The Gallup job-approval poll measures the number of people who approve or disapprove of the president during a particular week. Harry Truman and George W. Bush have had the most volatile approval ratings. Both presidents had approval ratings of more than 80 percent during their presidencies, but their ratings dropped below 30 percent approval before each left office.
From 1936 to 2012, Gallup presidential election polls have accurately forecast all but three results: Truman's victory in 1948, Jimmy Carter's 1976 win and Barack Obama's re-election in 2012.