How Does BC-STV Work?

The way that the Single Transferable Voting (STV) System calculates votes and moves votes around can seem a little complicated and intimidating at first blush. But the steps themselves are not really that complicated when you break them down; it’s more of a matter of repetition. At any rate, it doesn’t really matter since you don’t need to understand the calculations to know how to use STV itself, and using STV is really quite simple. I posted about a website that walks you through an election and even create your own.

Nevertheless, I still think it’s helpful to have an understanding of the broader principles that form the basis of STV. This is especially true when you consider that the majority of criticisms of STV rely on fear-mongering, half-truths or outright lies in order to deceive people into voting against it; having a bit of knowledge will help you recognize the valid criticisms from the dishonest ones.

So the following is an explanation that works around the nitty-gritty math and calculations that can overwhelm those who are unfamiliar with STV. It’s very much a simplification; but one that, hopefully, will better help you understand the underlying principles. So that you don’t just know what is happening but why as well.

The system can basically be summarized into a handleful of steps:

1. Voters rank the candidates on the ballot in the order of their preference (1,2,3,4 etc...) indicating "if my top choice gets eliminated or elected, my next choice is who I will vote for in the next round of voting"
2. Count the votes.
3. If none of the candidates has a majority share of the vote (eg. 50% + 1 for a single member riding), then eliminate the candidate with the fewest votes and transfer their votes to the next choice on the ballot. Repeat step two.
4. If a candidate has a majority share of the vote and there are still seats to elect, then eliminate the elected candidate and transfer their votes to the next choice on the ballot. Repeat step two.
5. Repeat the above until all seats have been filled.

If you can understand the above steps, then you can stop reading here. That's all you need to know. If you want a more detailed explanation, keep reading.

Explanation of BC-STV in a single-winner election.
Explanation of BC-STV in a multiple-winners election.