End-of-term approval ratings for presidents are valuable in forecasting voter preferences in the following election. The higher a president's job approval ratings are at the end of his term, the more likely it is a candidate from his party will succeed him in the White House.

That, of course, is not always the case. Democratic President Bill Clinton left office with a relatively high approval rating in 2000, but his impeachment during a second term harmed the chances that his vice president, Al Gore, would succeed him. Republican George W. Bush narrowly won the White House in the 2000 election, though he lost the popular vote.

President Barack Obama's sound approval rating may not be an indicator of Democrat Hillary Clinton's chances in 2016, either. The last time voters elected a Democrat to the White House after a president from the same party had just served a full term was in 1856, before the Civil War.