ASPEN — For the first time in Aspen’s history, city voters will rank their choices for mayor and City Council in order of preference on the May ballot.

The Aspen City Council adopted on Monday a method of instant runoff voting (IRV), which will be a hybrid of other systems used around the country. However, the method chosen has never been done before in the United States because Aspen elections involve electing multiple City Council candidates.

Council members last month put off their decision on what voting method should be chosen so the public could weigh in on all methods considered and the complexities surrounding them.

A task force largely made up of government officials and citizens was convened months ago and recently recommended a two-count, batch elimination method. But because of potential problems that could result in different outcomes based on mathematical probabilities, the task force spent the past couple of weeks further studying IRV methods. On Monday, the task force proposed a compromise, which the council adopted.

On May 5, Aspen voters will rank their choices for mayor and two council members. Mayor Mick Ireland’s seat will be open, as well as the seats of council members Jackie Kasabach and Jack Johnson. All are considering another bid but have not officially announced their candidacy.

The IRV method that has been chosen rests on a threshold of votes counted, which will be determined by Aspen City Clerk Kathryn Koch. All first and second rankings are counted, and the two candidates with the most total votes who have reached the threshold are elected.

If one candidate reaches a threshold, that person is elected, and the next two highest voter getters enter an instant runoff. The ballots are recounted with the highest ranking for the candidate still remaining, which is one vote counted for that individual. If no candidate receives the threshold in the initial tabulation, then the four highest vote getters enter the instant runoff. The candidate who receives the fewest votes will be eliminated, and the candidate with the most votes is elected.

The tabulation continues for the election of the second City Council seat. In this round, the three candidates who initially made the instant runoff but who were not elected in the first round continue on. Each ballot is recounted, and the highest ranking for a candidate who is continuing will be counted as a vote for that person. The candidates with the fewest votes will be eliminated, and so on until someone gets the most votes.

Jim True, the city’s special counsel, said that method assures that everyone has two votes counted.

A new computer program will have to be written to count the scanned ballots. The city government will pay Caleb Kleppner, a senior analyst who specializes in election administration, $7,500 to write the program and count the ballots on election night.

A hand count will occur afterward to build confidence in the untested system.

Koch, who has been overseeing Aspen city elections since the 1970s, said logic and accuracy tests of the count will be done in public. She acknowledged that the computer program will be written in an open source code that can be viewed by residents.

Some residents have suggested that the integrity of Aspen’s elections is in jeopardy and that there could be legal challenges to whatever outcome occurs in the May election.

“We feel very strongly that it will survive any legal challenge,” True said, adding he’s consulted with constitutional law experts, and they’ve told him challenges to other IRV methods around the country have not been successful.

“But I’m the last person to guarantee that because lawyers don’t guarantee things,” True said.

Koch said she has consulted with the head of elections at the Secretary of State’s Office, who has signed off on Aspen’s IRV method.

The council adopted the IRV method in response to a mandate by the majority of Aspen residents, who voted in November 2007 to eliminate runoff elections, which have required a June election if council candidates didn’t receive 45 percent plus one and if the mayor didn’t get 50 percent plus one of the vote in the May election.

Council members agreed that the IRV method they adopted most mimics Aspen’s traditional election method, but makes it more efficient and cost effective because it eliminates a June election.

Koch said she plans to educate the public on the new voting method through videos on GrassRoots TV and the city’s website, as well as posting sample ballots online and hosting community meetings.