Batch elimination is the practice of eliminating multiple candidates in a single round of a ranked choice voting tabulation. This quick explainer will help you understand what it is, when it is used, and why it matters -- or doesn’t matter.
Why Use Batch Elimination?
Batch elimination simplifies the tabulation process while preserving the benefits of ranked choice voting (RCV). It does not impact the way voters cast their ballots or the way candidates run their campaigns. It is simply a more concise way of counting the ballots.
Batch elimination was originally a way to simplify hand-counting of RCV ballots, but is still used in many U.S. RCV elections which use machine counting to simplify the results presentation.
What Circumstances Trigger Batch Elimination?
In RCV elections which allow batch elimination, multiple candidates in last place(s) will be eliminated in a single round if their combined vote total in that round is less than the vote total of any other candidate.
This could also be interpreted as the elimination of all candidates whose victory is mathematically impossible.
Can Batch Elimination Change the Outcome?
No. Batch elimination only occurs when batch elimination will not alter the outcome. See the two examples below for more details.
Which RCV Jurisdictions Use Batch Elimination?
Several U.S. cities and states use batch elimination for their RCV elections, including Maine, New York City, and Minneapolis.
Example A: Batch Elimination in Practice in Maine
Below are the results from Maine’s 2018 election for the 2nd congressional district. Batch elimination can be identified because two candidates, Hoar and Bond, are eliminated in the same round.
Figure 1: Official Results, 2018 Maine CD-2
Now, consider the case in which candidates had been eliminated one-at-a-time. Hoar would be eliminated first and their 2.4% of ballots transferred. Bond would be eliminated next, regardless of the outcome of the first round transfer. Even if Bond wins 100% of the transfers from Hoar, Bond’s vote total will still be only 8.1%, not enough to advance to the next round.
Thus, Hoar and Bond are mathematically certain to be the first two candidates eliminated because their combined total in the first round is smaller than that of every other candidate. They can be eliminated in the same round without impacting the total.
Example B: Batch Elimination Not Permitted
In this example, we will illustrate a scenario where candidates must be eliminated one-at-a-time.
Figure 2: First-choice results from Portland ME 2020 City Council District 5
In this first round, batch elimination cannot be used on the bottom two candidates because it is not yet mathematically impossible for Coyne to move into second place and then win the election. Or, in other words, the total votes between Capron and Coyne is greater than the total of at least one other candidate, so they cannot be eliminated as a batch. In the election above, it is clear that Kenneth Capron finishes in last place. However, the 682 ballots which selected Capron as first choice have the potential to change what happens next. If all or most ballots were transferred to John Coyne, Coyne could move into second place and be protected against elimination for another round. If not, Coyne will be eliminated next.
Below are the full round-by-round results.
Figure 3: Round-by-round results from Portland ME 2020 City Council District 5
Ultimately, batch elimination is simply a tool for making the RCV tabulation more concise and digestible. It will never impact the outcome of the election, but can nevertheless be helpful to understand.