Multi-winner RCV is different from single-winer RCV

This Washington Post story is an important cautionary tale. In Arlington County, Virginia, the Democratic primary for two seats on the county board is using a multi-winner form of RCV (a system when employed elsewhere is sometimes called the “single transferable vote” or STV): “Voters are able to rank up to three candidates on their ballots in the race for two open spots on the party’s ticket.”

Implementation apparently has left something to be desired:

But there seems to be one hiccup so far: Not many people understand how it works.

“I don’t know that it’s has been properly communicated to the community,” said Kevin Saucedo-Broach, a former candidate in the Democratic primary for an Arlington-based seat in the House of Delegates. “We’re just going to let people walk into the polls and kind of hope for the best? That’s not even the bare minimum.”

Saucedo-Broach said he is not opposed to the idea on paper. Like many others, he hopes the system might reduce political polarization and offer more representation for minority blocs.

But he is one of several Arlington politicos who say local and state government officials haven’t conducted enough outreach to educate voters about the new process — a call that’s coming from an unusually wide mix of people.

Multi-winner RCV is much more complicated mathematically than single-winner RCV. As someone who has spent a lot of time studying RCV, I confess that I’ve had considerable difficulty wrapping my mind around the mathematics of multi-winner versions. Apparently, according to this story, I’m not alone:

Much of the frustration has focused on the wonky, hard-to-follow way that votes are counted: Because ranked-choice voting is being used to pick not one but two nominees, critics say the tabulation methods are unfamiliar, confusing or even undemocratic.‘ …

Because multiple candidates are being selected, some of them may win the necessary number of votes before others. If a candidate reaches this threshold before anyone else, only a fraction of the ballot that ranked the winning candidate first is passed onto the voter’s second (or third) choice.

Also, for those interested in this topic, I renew my previous recommendation of Jack Santucci’s book on the use (and eventual disuse) of STV in municipal elections in the United States. One key point of the book is that the mathematical features of STV cause it to operate differently from other forms of proportional representation, and these features can cause problems for parties and interest group coalitions in their competition for seats on a multi-member city council. (Very briefly, because it’s a technical point, and one should read the book for a full explanation: lower-ranked candidates on a voter’s ballot may cross over to a rival party’s or coalition’s candidate, causing a leakage of support for the voter’s main partisan or coalitional preference, and this kind of leakage can gum up the predictability of the electoral competition, thereby causing across-the-board dissatisfaction. The mathematics of other PR systems don’t have the same leakage effect; nor, by the way, does the “self-districting system” I’ve proposed.) The book marshals data in support of an argument that the particular methodology of STV was one factor that caused its widespread repeal after many municipalities began to experiment with the system in the Progressive Era.

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