First of all, what is Instant Runoff Voting?
It is a voting method where voters casting a ballot in races with more than two candidates mark a first and (if desired) second and third choice for each office. It is a tally and elimination scheme, retallying without revoting (reallocating), and repeating until a majority of votes are reshuffled into one pile.
Why are we even talking about IRV and North Carolina? North Carolina adopted an instant runoff voting "pilot program for use in the 2007 and 2008 elections for local offices in volunteering jurisdictions. The pilot program instructed the State Board of Elections to select up to 10 cities for the 2007 elections and up to 10 counties for the 2008 elections. Two cities, Hendersonville and Cary, participated in the 2007 experiment. No counties volunteered in 2008, and the experiment expired. The bulk of the voter education was provided pro bono by non profit groups and proponents who were advocating for IRV.
Dr. Rebecca Mercuri, Internationally respected computer scientist and e-voting expert warns about risks -
"IRV and other proportional balloting methods have been proven to incentivize the introduction of electronic ballot tabulation in places where none previously was needed or has existed, and they further complicate what has become an increasingly closed process for the determination of election results....Since these methods lend themselves to potential 'gaming' of the ballot set that may not be independently detectable or auditable, these run-off styles must be prohibited."
Why do we say IRV is hard to count?
Unlike regular voting methods, with IRV, counting is not a matter of simply totalling up the 2nd or 3rd rounds. With IRV, votes may need to be transferred according to which candidates are eliminated in each round. A more detailed explanation follows from the Election Method website Minguo:
IRV does not comply with the summability criterion. In the IRV system, a count can be maintained of identical votes, but votes do not correspond to a summable array. The total possible number of unique votes grows factorially with the number of candidates. The larger the number of candidates, the more error-prone and less practical it becomes to maintain counts of each possible unique vote. It becomes impractical with more than about six candidates.
Suppose, for example, that the number of candidates is ten. In our current plurality system, the votes at any level (precinct, county, state, or national) can be ompressed into a list of ten numbers...In an IRV system, however, the number of possible nique votes is over ten factorial -- a huge number.
Under IRV, therefore, every individual vote (rank list) must be available at a central location to determine the winner. In a major public election, that could be millions or even tens of millions of votes. The votes cannot be compressed by summing as in other election methods because votes may need to be transferred according to which candidates are eliminated in each round.
IRV therefore requires far more data transfer and storage than the other methods. Modern networking and computer technology can handle it, but that is beside the point. The biggest challenge in using computers for public elections will always be security and integrity. If many thousands of times more data needs to be transferred and stored, verification becomes more difficult and the potential for fraudulent tampering becomes substantially greater.
To illustrate this point, consider the verification of a vote tally for a national office. In our current plurality system, each precinct verifies its vote count. The counts for each precinct in a county can then be added to determine the county totals, and anyone with a calculator or computer can verify that the totals are correct. The same process is then repeated at the state level and the national level.
The point is that once the votes are verified at the lowest (precinct) level, the numbers are available to anyone for independent verification, and election officials could never get away with "fudging" the numbers. At the lowest level, ballot problems such as "hanging chads" could be a problem, but adding the vote counts will certainly not be a problem. And this applies not only to conventional plurality elections, it applies also to Condorcet, Approval, and even Borda -- but not IRV
In an IRV election, the voting data cannot be "compressed" by adding the vote totals together at each level, so verification of the tally results becomes nearly impossible. The final result depends on all the votes, but even if the individual votes are all counted correctly, nobody can verify that the total pool of votes has not been tampered with at some level of the tallying process. And with IRV's erratic properties, someone could lower the rankings of a candidate to make him win or raise the rankings of a candidate to make him lose. It's a prescription for disaster and voter cynicism.
If IRV were superior otherwise, then its failure to comply with the summability criterion might be excusable. But IRV has been shown to fail with respect to every one of the criteria listed, including such basic criteria as monotonicity. To accept the additional security risks that IRV poses would therefore be the epitome of folly.