Individual polls can vary to a maddening degree, but the average of final national polls tallied at the RealClearPolitics.com website has been fairly accurate in the past three presidential elections.

2004
Leader in final individual polls: Bush 10, Kerry 2, tie 2
Average of final polls: Bush + 1.5 percent
Final result: Bush + 2.4 percent
Difference: 0.9 percent
See the 2004 polls here

2008
Leader in final individual polls: Obama 15, McCain 0
Average of final polls: Obama + 7.6 percent
Final result: Obama + 7.3 percent
Difference: 0.3 percent
See the 2008 polls here

2012
Leader in final individual polls: Obama 4, Romney 2, tie 3
Average of polls: Obama + 0.7 percent
Final result: Obama + 3.9 percent
Difference: 3.2 percent
See the 2012 polls here

2016
Leader in final individual polls: Clinton 11, Trump 1,
Average of polls: Clinton + 2.9 percent
SOURCE: Real Clear Politics

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