The principle behind vote auditing is simple: Humans should be double-checking machines. Audits don't necessarily assume that vote results have been tampered with -- they're designed to ensure votes haven't been miscounted and assure voters that the time they spend casting a vote is well worth it. The closer the race, the more important the audit. For example, New Mexico state law calls for a recount on federal- and state-level races where the victory margin is less than one-half of 1 percent [source: Verified Voting Foundation]. After concerns about vote rigging and fraud in a 2009 Afghanistan election, an audit of the election led to a run-off between President Hamid Karzai and Abdullah Abdullah [source: NY Times].
In the 2000 Florida election, auditing took the form of painstaking recounts of punchcard ballots. Even recounts can't be perfect -- the auditors had to deal with improperly punched cards and determine what votes should be counted -- but a physical ballot can typically be re-checked to verify election results. That's why some organizations concerned with voting security advocate that all direct recording electronic (DRE) voting systems use a voter-verifiable paper audit trail (VVPAT).
A VVPAT records a voter's interaction with an electronic voting machine on paper. Though this limits the environmental benefit of an electronic ballot, it does provide a backup copy of a vote that can be used if an audit is necessary. In the U.S., 31 states have enacted laws requiring paper ballots as of 2009 [source: Verified Voting]. But not all states offer DRE machines with paper ballot backups. As of 2010, 14 states offered DRE machines with VVPATs [source: Verified Voting].
That number should grow as DRE machines continue to replace older voting systems. The statistics also illustrate the variety of voting systems used in modern elections, even within a single country. On the next page, we'll dive into some differences in voting system hardware.