This question was posed on behalf of Alok Shukla, Election Commission of India.

The question

Which countries use a voter verified paper audit trail along with e-voting?

What are the different types of technologies used and what has been the experience so far? Are there any problems?

Summary of responses

The following countries are given as examples where a voter verified paper audit trail has been used or trialled in conjunction with e-voting: Venezuela, USA (North Carolina), Russia, Italy (autonomous province of Trento), Argentina, Israel and the Philippines.

Two specific technologies are mentioned. First, paper ballots with RFID chips embedded in them. The vote is cast via a computer and the voter’s choice then printed out, as well as being stored on the ballot’s chip. Votes can then be counted quickly by machines which read the chips. The second technology is Precinct Count Optical Scanners (PCOS), whereby the voter marks a machine-readable ballot paper and feeds it into the PCOS, which counts the vote electronically. The ballot then serves as a paper audit trail.

Considerations include ensuring that the voter does not receive the paper record of their vote, physical issues related to printing, and having paper receipts be single entities.

Experiences of using the paper trail as a manual audit, or recount, to corroborate any electronic counting have been mixed: in the example of the RFID system, data on the printed ballot supersedes the electronic chip; random audits of the PCOS paper ballots proved logistically challenging in the Philippines; and in North Carolina discrepancies between the electronic and paper counts have been due to human errors of the paper count.

Examples of related ACE Articles and Resources
Encyclopaedia:
• Auditing of e-voting systems

• Focus on e-voting 

• Electronic voting systems

Names of contributors
1. Robert Krimmer
2. Charles Winfree
3. Ingo Boltz
4. Peter Wolf

examples include Venezuela, several states in the US, Russian prototype, Autonomous province of trento/italy etc.

when designing VVAT make sure, the receipts are single entities, i.e. not printed on a consecutive roll (otherwise a sequence of votes might allow to breach voter secrecy). 

make somehow sure that the vvat's cannot be taken away (control over the process), i.e. could be shown behind a glass and then dropped after display

usability is key ...

determine in the electoral code what is the authorative vote - the VVAT (recommended) or the electronic vote

These are just some quick thoughts, i'm sure others can add a lot more.

The direct record equipment in North Carolina, and several other US states, employs the paper audit trail. 

Though having the paper increases confidence in the election, and might deter tampering, there are several issues:

1.  It is imperative that the voter not receive the paper indicating how they vote; this would open the door to vote buying ( "I'll pay you $20 for every slip you bring me showing that you voted for X")

2. Voters seldom actually review the paper.

3.  The printer, with its moving parts, is most unreliable part of the equipment.

4.  It is inevitable that there will be paper jams, nobody notices that the printer ran out of paper, someone puts the roll in backwards, the ink runs low and makes the receipt difficult to read.

5.  We have audits after every election.  Whenever there has been a discrepancy between the "electronic tally" and the tally produced by review of the audit slips, it has been the result of errors in the manual review of the audit trail.  This is partly because the paper trail records every action by the voter, with the result that if the voter changes his or her mind before casting the ballot, there would be several entries that can be easily misinterpreted by the auditor.

Argentina and Israel have trialled systems that use RFID chips embedded in paper ballots. A computer guides voter choices, but doesn't store the vote internally. Instead, it prints it (human readable) on a paper ballot, and ALSO writes it electronically and contact-less on an RFID chip embedded in the paper ballot. 

Counting happens by reading out the RFID chips of all ballots in the ballot box - not quite "instant" as with HDD/Flash storage on the machine but still fast, but much more transparent as party observers (or even voters themselves) can use their own cheap standard RFID readers to sample consistency between printed vote and RFID stored vote.

In case of recount, what's printed on the ballot outweighs what's stored on RFID since the voter has visually and without aids verified it. 

The Philippines used PCOS (Precinct Count Optical Scanners) machines and OMR (Optical Mark Reading) ballots throughout the country for the 2010 elections. With this system the voter marks a machine readable ballot paper and feeds it through the PCOS into the ballot box. The PCOS counts the votes electronically and the paper ballots establish a physical audit trail that can be recounted as required.

The added value of a paper trail is that properly conducted audits with manual recounts in randomly selected polling stations can verify the electronically established results.

Organizing such an audit and recount exercise can be challenging. In the Philippines the random sampling was planned to be televised live on the afternoon of the Election Day. After the broadcast it turned out that the sampling process was flawed and had to be repeated, this time without TV coverage. Notifying the audit teams was additionally delayed due to communication problems.

The manual audits were to be conducted immediately after the closing of the polling stations and the audit teams should not have been aware of the electronic results. Due to the delays many audits happened only days later, with the electronic results readily available for the auditors.