How does STV work?

(Warning : A bit of maths but you can skip that bit )

All these posts grouping current constituencies into 3s, 4s, and 5s must be leading somewhere, right? Well, yes, and this is where I explain further.

The Single Transferable Vote (STV) system works on the basis of multi-member constituencies. Since I’m not proposing to increase the number of MPs in the House of Commons, this means fewer, but larger, constituencies.

It is possible to run something like STV while keeping one MP per constituency. This is the Alternative Vote (AV) system that was put to the British people in a referendum in 2011. As we know, that proposal was defeated, but I say more about that referendum in this post.

AV is a simplified form of ATV, but it produces different results. Importantly, it doesn’t produce proportional results. In fact, it can be less proportional than FPTP.

STV doesn’t exactly aim to produce a proportional result—but it is a fortunate by-product. Other systems, which I’ll discuss in later posts, have proportionality built in.

Proportionality aims to be fair to parties, but as I said in a recent post, I believe fairness to voters is a more important attribute.

STV reflects how many of us think while planning our vote. Maybe there’s an independent candidate named Molly who is your neighbour and has good ideas. She hasn’t stood before, but you’d like to give her a chance. However, you also know that if you vote for her and she doesn’t garner enough support, you won’t have any say in the contest between the sitting MP, who has done a decent job, and their main challenger, whom you don’t like. STV allows you to have both.

So this is how it works.

All the candidates are listed on the ballot as before, but because of the multiple vacancies, parties aiming for multiple success will field more candidates. This will mean a longer ballot paper, but the extra reading time will pay off with extra choices.

Voters then order their preferences by marking 1, 2, or 3 next to the candidates of their choice.

Initially, all ballot papers are sorted based on first preference only. At this stage, second and subsequent choices do not count, similar to FPTP. Every ballot counts as a single vote. That’s the S in the STV.

But here’s where STV differs: when the count is done, the candidate with the most votes isn’t necessarily elected outright. They must reach a threshold number of votes.

OK, this is where it get’s technical…

The threshold is calculated using the formula is:

How the STV Process Works

The chance of a candidate being elected on the first count is quite low, though it is possible if the strongest party has an outstanding candidate.

More commonly, the candidate with the fewest votes is eliminated in the first round. In some cases, more than one candidate might be eliminated early on if it’s clear they won’t be able to bridge the gap to reach the threshold.

All votes currently in the piles for eliminated candidates are then transferred to the next preference listed on each ballot paper. This is the "T" part of STV—Transferable.

Votes are counted again, and it’s reassessed to see if any candidate has reached the threshold.

In 2011, Opponents of AV (which also includes this transfer process) claimed some people were getting more votes - but this is not so. We all get one vote - its assumed that if your preferred candidate is till in the running, you’d still vote for them.

This process continues until a candidate reaches the threshold and is elected. Then STV gets even more interesting—it doesn’t just cater to votes ‘wasted’ on an unsuccessful candidate; it also accounts for votes that aren’t needed by a successful candidate.

For example, if the quota is 800 votes and at the end of a round, Molly Neighbour has 1,000 votes, her surplus of 200 votes is also transferred. Since we can’t know which specific 200 votes are the surplus, all 1,000 votes are transferred, but at a reduced value—1/5 in this case, which is the surplus divided by the total number of votes.

In AV, only one person is elected so this reduced vote system isn’t needed, but under STV there may still be vacancies to fill.

This process continues, with each round either electing a candidate or eliminating one (or more, though multiple eliminations are rarer in later rounds). The process ends when the correct number of candidates has been elected.

That, then is the system. Changing to STV would have repercussions, and no system is perfect. I’ll cover some of those in a future post.

Many more discussion pieces to come

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