Approximately 24 million—one of every eight—active voter registrations in the United States are no longer valid or are significantly inaccurate.
More than 1.8 million deceased individuals are listed as active voters.
Approximately 2.75 million people have active registrations in more than one state.
Meanwhile, researchers estimate at least 51 million eligible U.S. citizens are unregistered, or more than 24 percent of the eligible population.
http://www.pewcenteronthestates.org/uploadedFiles/Pew_Upgrading_Voter_Registration.pdf