The competition for the nomination of the Democratic Mayor of New York City is wide open. It’s the kind of race that a ranking vote is designed to help with, with voters backing their top poll without losing the opportunity to weigh the most suitable candidates.

It’s also the type of race that could test one of the main risks of a ranked poll: a phenomenon known as ballot exhaustion. A ballot is considered “exhausted” when every candidate classified by a voter has been eliminated and this ballot is no longer included in the election.

With so many viable candidates and most New Yorkers first-time ranking polls, all the ingredients for a large number of depleted ballots are in place. If the race is close enough, it is a factor that could even decide the choice.

This possibility does not necessarily mean that New Yorkers are worse off when it comes to voting according to the rankings. However, the risk of exhaustion of the ballot paper is an underestimated reason why the alleged advantages are not always recognized when voting by ranking.

Cities and other local governments have polled eight states and across Maine nationwide. It will be used for the first time this year in the New York Mayor’s Race, allowing voters to rate up to five candidates in their order of preference.

If no candidate receives a majority of the first preferential votes, the race is decided by an immediate runoff: the candidate with the fewest votes in first place is eliminated, and the votes of those who preferred the eliminated candidate are voted on the second of these voters transfer decisions. The process continues until a candidate wins a majority of the remaining ballots.

However, such a system is complicated. It urges voters to use a new and unusual set of rules to make many more decisions than they would normally have to make. As a result, many will not rate the maximum number of candidates. There is a possibility that the election result will be different if every voter has filled out a complete voting slip.

A recent poll by the Manhattan Institute / Public Opinion Strategies found evidence that ballot exhaustion could be a major factor in New York’s mayoral elections. The poll, which asked voters to complete the full ballot, found that Eric Adams led Andrew Yang by 52 to 48 percent in a simulated instant runoff election. Behind the top scores lurked a group of 23 percent of respondents who had rated some candidates but had not rated Mr. Yang or Mr. Adams. If these voters had preferred Mr. Yang, the poll might have turned out differently.

A fatigue rate of 23 percent would be pretty high, but not without precedent. In the 2011 San Francisco Mayor’s Race, 27 percent of the ballots were neither of the two candidates who made it to the finals. And, on average, 12 percent of the ballots in the three special city council elections held in New York City this year were exhausted.

Even a lower percentage of depleted ballots can make a difference in a tight race. An analogous case is the special mayoral election in San Francisco in 2018, in which London Breed prevailed by just under one percentage point. In that race, 9 percent of the ballots rated neither Ms. Breed nor runner-up Mark Leno.

It’s impossible to know for sure, but there are plausible reasons to believe that if each voter had chosen one of the two final candidates, Mr. Leno would have won the election. Mr. Leno, for example, won broadcast votes – those cast by voters who did not select either Ms. Breed or Mr. Leno as their first choice – by a margin of 69 to 31 percent; he would have won if the exhausted ballots had expressed a similar preference.

The large number of depleted ballots in ranked elections might come as a bit of a surprise as the format is designed to ensure voters don’t waste their ballots by supporting non-viable candidates. In the archetypal case, the choice of rank could allow voters to endorse a small party candidate like Ralph Nader without the risk of jeopardizing their preferred large party candidate, whom they could safely move to second place.

Voters, however, do not always have the same degree of clarity about which candidates will make the final round of voting as they would have in the 2000 presidential election when Mr Nader finished third as the Green candidate with almost three million votes. Even without eligibility to vote, the primaries often have flowing multi-candidate areas where clear favorites in the general election are nowhere near as obvious as a Democrat versus a Republican.

Fortunately, ranking voting tends to increase the number of options available to voters and tarnish what may otherwise be a relatively clear final choice. Interest groups and ideological factions have less incentive to group behind a single candidate in a ranked election, knowing that their voters can still group behind a single candidate on election day.

Partly as a result, the number of depleted ballots is highest in wide-open races, where voters have the least clarity about the likely endgame.

In the three ranked special elections for New York City Council seats, the number of ballots exhausted was higher in races without a strong first-round candidate. For example, when the top candidate had only 28 percent of the vote in the first ballot in the 15th district, 18 percent of voters had not rated either of the two best candidates.

Understand the NYC Mayoral Race

    • Who is running for mayor? There are more than a dozen people in the running to become New York’s next mayor, and the primary is on June 22nd. Here is an overview of the candidates.
    • Get to know the candidates: We’ve hired leading mayoral candidates on everything from police reform and climate change to their preferred bagel order and training routine.
    • What is a ranking poll? New York City started voting in the primary this year, and voters can list up to five candidates in order of preference. Confused? We can help.

In the mayor’s primary today, the New York Democrats can’t be sure whether there will likely be a final matchup. There are currently 13 Democratic candidates in the running, at least five of whom can be considered front runners. Andrew Yang, the lead polling candidate for most of the year, has declined in recent polls; others, like Kathryn Garcia, seem to be on the rise. With so much uncertainty, even political junkies may not be entirely sure whether their vote will have an impact in the finals.

Voters who are not political junkies have a very different challenge. Voting according to the ranking is demanding. Voters have to make informed judgments about many more candidates than they would otherwise. Less informed voters are less likely to make such judgments and are therefore less likely to rate the maximum number of candidates, which increases the likelihood that they will not list either of the last two candidates on the ballot.

Other voters may not fully understand how ranking works. In a NY1 / Ipsos poll in April, only 53 percent of likely voters said they were very familiar with the ranking and 28 percent said they were uncomfortable with it.

According to a 2004 study by the Public Research Institute, only 36 percent of San Francisco voters who did not fully understand the ranking rated the maximum number of candidates in the 2004 mayoral contest, compared with 63 percent of those who said at least they did understood pretty well.

In order to take full advantage of the leaderboard choice, voters need to know something that is often not given: it works through the instant drain. This may seem obvious, but it is not mentioned on the ballot, it is not mentioned in the educational material sent by the city (and received at my address), and it is not highlighted on the city’s election website. There isn’t even an explanation as to why candidates are ranked.

Without an explanation of how their ballots affect election results, voters may not understand why it is in their best interest to rate the maximum number of candidates.