How do votes turn into seats in Tasmania?

You need a quota to win a seat in a Tasmanian House of Assembly election. But in practice no-one polls exactly a quota. In reality, the final seats are decided by partial quotas and the distribution of preferences.

Yet these patterns are not random – parties that poll over a certain vote share are much more likely to win a certain number of seats, and parties that poll below a certain vote share have little chance of winning that last seat. Given a certain vote share, some parties are more likely to win a seat on that vote than others, and those trends have shifted over time.

This blog post is an update to my 2024 blog post, published prior to that election. That election produced some interesting results that deviated from previous trends.

This first chart plots out the number of quotas polled per party and the number of seats won from 2006 to 2024. It’s worth noting that there are always more quotas available than seats – almost 30 quotas from 2006 to 2021, and almost 40 quotas in 2024, compared to 25 or 35 seats respectively. The chart only shows the results for Labor, Liberal, Greens and JLN. In practice, until 2024, the ‘others’ vote was so scattered that they pretty much never won a seat (apart from Kristie Johnston in 2021). But they weren’t a single unit, so it’s not a fair comparison.

There is a very clear relationship between quotas polled and seats won. Most of the time parties win more quotas than their seat share, which makes sense considering that there are more quotas than seats available.

The 2024 results are marked with stars and stand out. Prior to 2024, there was only one case during this time period of a party winning more seats than the quotas they polled – Labor in 2021 won nine seats with 8.4 quotas. But in 2024, all four of these parties polled fewer quotas than their seat share. This likely reflects the dispersed vote amongst numerous other independents who didn’t win.

The 2024 Liberal result was clearly the most efficient result for a party in the last twenty years of Tasmanian elections. The Liberals polled just 12.3 quotas but won fourteen seats.

This next chart does the same but at the level of each electorate. The equivalent chart last time just covered the three big parties but now I’ve included JLN and all independent or minor party tickets that polled over a quarter of a quota.

It clearly visualises the range of results that is likely to produce a particular seat count.

For example, every ticket that polled over 0.7 quotas won a seat. Groups that poll over 1.5 quotas are more likely to win two seats than one, but that doesn’t always happen.

There is quite a lot of overlap, with vote shares that have sometimes elected an extra seat, and sometimes not.

The patterns by party also vary. Liberal tickets have often required a higher vote for the same seat count as Labor.

The other thing that stands out is that the 2024 results (marked as stars) generally had some of the lowest share of quotas to produce a particular seat result in the period analysed. No party had won two seats from less than 1.5 quotas before the Greens did it in Clark in 2024.

No Greens ticket has managed to win a seat with less than 0.7 quotas (at least since 2006), but Garland, O’Byrne, two of three JLN winners and Johnston have managed to do so in 2024. Johnston also did it in 2021.

The Liberal Party managed to win three seats in Bass and Franklin with a vote which would have previously only elected two members.

Interestingly, the Labor results don’t show the same trend. They did not benefit from the ability to win seats off smaller vote shares than expected.

This next chart takes the same data as the first chart, but shows it in a different way. It compares the number of seats won for each of the three main parties to the number of quotas they polled.

Labor in 2018 and 2021 had much more efficient results than the other parties, but in 2024 the others caught up, with the Liberal Party doing much better. Whatever preference flow that had previously benefited Labor seems to have changed in 2024, as the field of other parties has grown.

For the rest of this post, I wanted to drill further into what could be causing this shift. To do this, I wanted to look at how many votes each party gained from other parties throughout each preference count, and how many votes were lost from the party ticket (or leaked) as candidates are knocked out of the count. Candidates have been grouped as Labor, Liberal, Greens, JLN and others.

Prior to 2024, Labor tended to gain more votes and lose fewer votes.

I have identified the 2024 results as stars. Labor tickets tended to leak further votes in 2024 than the Liberal Party, but in general tickets have tended to both gain and leak more votes, which reflects the increased complexity of the ballot paper leading to more votes moving around.

The next chart groups the data by party and by election year, just showing the gaining of preferences.

In 2024, the Labor tickets gained fewer preferences, while Liberal and Greens both increased their gaining of preferences.

Overall the 2024 election saw preferences playing a bigger role in general. The make-up of those preferences also became more favourable to the Greens and Liberal Party, with more conservative preferences and more independent preferences, and thus made the system less favourable to the ALP.

This is a good reminder that the application of electoral systems do not permanently favour one party over another – a change in voting patterns can change who benefits.

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