Election Science
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Election Science

Strategy with STAR voting and IRV

With multi-round voting methods, there are two fundamental types of strategy: compromise and pushover.

Compromise means helping to advance someone you like less but who has a greater chance of beating a competitor you like even less in the next round. Think of it like voting Democrat instead of Green so the Democrat makes it to the runoff with the Republican and has a better chance of winning than the Green would have. (My aunt in Iowa did this when she voted for Joe Biden in the 2020 Democratic primary even though she preferred Elizabeth Warren, since she thought Biden would have a better chance to beat GOP nominee Donald Trump.)

Pushover means doing the opposite—helping advance a weak candidate against a candidate you prefer. E.g. a Republican voting Green because he’s sure the Republican will make it to the runoff, but the race is close between the Green and Democrat.

Instant Runoff Voting (the most well known form of ranked voting, and thus widely referred to generically as “ranked choice voting”) is vulnerable to both. You can imagine my aunt ranking Biden as her first choice with IRV, even though Warren was her sincere favorite. Here’s a great video explanation by Andy Jennings, a co-founder of the Center for Election Science who did his math PhD thesis on voting methods.

What do these strategies look like with STAR voting?

Compromise

You prefer X over Y over Z. The total scores favor X over Z over Y, and the ranked preferences favor Y over Z over X. Z (your least favorite) is going to beat X, but if you give Y a 5 and X a 0, then you might switch the top two finalists to X and Y instead of X and Z, which will cause Y to win. You get your #2 instead of your #3.

Pushover

You prefer X over Y over Z. The total scores favor X over Y over Z, and the ranked preferences favor Y over X and Z. Y (your second favorite) is going to beat X, but if you give Z a 5 and Y a 0, then you might switch the top two finalists to X and Z instead of X and Y, which will cause X to win instead. You get your #1 instead of your #2.

Plausibility

The difference with STAR voting is that these strategies are virtually impossible, because the scoring process obviates the vote splitting problem that allows these issues with IRV. Compromise with IRV involves trying advance a candidate who is broadly strong but narrowly weak due to vote splitting (e.g. a moderate Dem) instead of a candidate who is narrowly strong but broadly weak due to vote splitting (e.g. a Green). Pushover with IRV involves trying to advance a candidate who is narrowly strong but broadly weak due to vote splitting (e.g. a Green) instead of a candidate who is broadly strong but narrowly weak due to vote splitting (e.g. a moderate Dem).

This vote splitting issue is only a factor with IRV because it is effectively just a round-by-round version of plurality voting, where you can only vote for one candidate at a time! STAR voting eliminates this problem by allowing voters to score each candidate independently and simultaneously. As such, it is no great surprise that STAR voting outperforms IRV with any mixture of honest or tactical voters according to voter satisfaction efficiency.

A graph of voter satisfaction efficiency by Dr. Jameson Quinn, Harvard stats PhD and former board member with the Center for Election Science

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Clay Shentrup

Clay Shentrup

advocate of score voting and approval voting. software engineer. father. husband. american.