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Meridiem 2021

A model of the upcoming 2021 Western Australia state election using various inputs such as voting intention polls and historical performance to model the election. We then run 50 000 simulations to forecast the election. Full technical details of how the model works >>

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Labor is clearly favoured to win a majority

This is our forecast for the 2-party-preferred (2pp) vote. The probability of Labor’s 2pp vote ending up in each of the above areas is described by the legend above the chart (so for example, there’s a 50% chance that Labor’s 2pp vote ends up in the grey area).

We currently estimate that Labor will win between 58.7% and 68.8% of the two-party-preferred (2pp) vote on election day. This range will likely narrow as election day approaches.


Seat totals forecast

The forecasted distribution of seats for each party/grouping. Higher bars indicate outcomes which happened more frequently in our simulations.

Dashed line drawn at the required number of seats for a majority in the Legislative Assembly (30)

The range of seat estimates for all of the above parties/groupings:

LowerMedianUpperOrder
Labor4350551
Liberals15112
Nationals2473
Greens0004
One Nation0015
Others0006

Labor vs Liberals/Nationals seat margin forecast

The forecasted distribution of (Lib/Nat seats – Labor seats), which is usually a decent measure of the chance of either side forming government.

Higher bars show outcomes which occurred more often in our simulations.


Probability of various outcomes

Numbers may not sum to 100 due to rounding.

OutcomeProbability
1Labor wins an outright majority> 99 in 100
2The Liberals win an outright majority< 1 in 100
3Labor requires the Greens for a majority< 1 in 100
4The Liberals require the Nationals for a majority< 1 in 100
5No party or coalition has a majority< 1 in 100
OutcomeProbability
6Labor holds the most seats> 99 in 100
7Liberals + Nationals hold the most seats< 1 in 100
8Neither Labor nor the Liberals/Nationals have a plurality< 1 in 100


How the forecast has changed over time

The figures above refer to how many times each outcome occurred in every 100 simulations. Numbers may not sum to 100 due to rounding.


Seat-by-seat probabilistic forecast

This table shows how often each party/grouping won each electorate for every 100 simulations. Numbers may not sum to 100 due to rounding.

Note: not a forecast of the vote each party is expected to win in each electorate.

DistrictLaborLiberalNationalGreenOne NationOthers
Albany99< 1< 1< 1< 1< 1
Armadale> 99< 1< 1< 1< 1< 1
Balcatta> 99< 1< 1< 1< 1< 1
Baldivis> 99< 1< 1< 1< 1< 1
Bassendean> 99< 1< 1< 1< 1< 1
Bateman5644< 1< 1< 1< 1
Belmont> 99< 1< 1< 1< 1< 1
Bicton> 99< 1< 1< 1< 1< 1
Bunbury> 99< 1< 1< 1< 1< 1
Burns Beach> 99< 1< 1< 1< 1< 1
Butler> 99< 1< 1< 1< 1< 1
Cannington> 99< 1< 1< 1< 1< 1
Carine3763< 1< 1< 1< 1
Central Wheatbelt1< 199< 1< 1< 1
Churchlands3367< 1< 1< 1< 1
Cockburn> 99< 1< 1< 1< 1< 1
Collie-Preston> 99< 1< 1< 1< 1< 1
Cottesloe1684< 1< 1< 1< 1
Darling Range> 99< 1< 1< 1< 1< 1
Dawesville8911< 1< 1< 1< 1
Forrestfield> 99< 1< 1< 1< 1< 1
Fremantle> 99< 1< 1< 1< 1< 1
Geraldton78138< 1< 1< 1
Hillarys982< 1< 1< 1< 1
Jandakot991< 1< 1< 1< 1
Joondalup991< 1< 1< 1< 1
Kalamunda> 99< 1< 1< 1< 1< 1
Kalgoorlie67266< 12< 1
Kimberly> 99< 1< 1< 1< 1< 1
Kingsley991< 1< 1< 1< 1
Kwinana> 99< 1< 1< 1< 1< 1
Landsdale> 99< 1< 1< 1< 1< 1
Mandurah> 99< 1< 1< 1< 1< 1
Maylands> 99< 1< 1< 1< 1< 1
Midland> 99< 1< 1< 1< 1< 1
Mirrabooka> 99< 1< 1< 1< 1< 1
Moore3392< 11< 1
Morley> 99< 1< 1< 1< 1< 1
Mount Lawley> 99< 1< 1< 1< 1< 1
Murray-Wellington991< 1< 1< 1< 1
Nedlands4951< 1< 1< 1< 1
North West Central52345< 11< 1
Perth> 99< 1< 1< 1< 1< 1
Pilbara98< 11< 1< 1< 1
Riverton937< 1< 1< 1< 1
Rockingham> 99< 1< 1< 1< 1< 1
Roe< 1595< 1< 1< 1
Scarborough7525< 1< 1< 1< 1
South Perth6634< 1< 1< 1< 1
Southern River> 99< 1< 1< 1< 1< 1
Swan Hills> 99< 1< 1< 1< 1< 1
Thornlie> 99< 1< 1< 1< 1< 1
Vasse1386< 1< 1< 1< 1
Victoria Park> 99< 1< 1< 1< 1< 1
Wanneroo> 99< 1< 1< 1< 1< 1
Warnbro> 99< 1< 1< 1< 1< 1
Warren-Blackwood301681< 1< 1
West Swan> 99< 1< 1< 1< 1< 1
Willagee> 99< 1< 1< 1< 1< 1

Most likely median seats (on a two-party-preferred basis)

If you lined up all the districts from highest Labor 2pp to highest Liberal/National 2pp, the median district would be the district right in the middle of this lineup. If all districts are won by either Labor or the Liberals/Nationals, then the median district effectively decides which side forms government. This is very similar, though not identical, to the American concept of a tipping-point state (our median district calculation only looks at which district is in the middle of the pack in 2pp terms, without accounting for wins by minor parties/independents).

The table below shows the ten most likely median districts on a 2pp basis, and how frequently they end up as the median district in our simulations. Note that we estimate that there is about a 2 in 5 chance that the median district will be one of the 49 not listed below.

DistrictProbability of being the median district
Southern River8 in 100
Landsdale8 in 100
Burns Beach7 in 100
Darling Range7 in 100
Wanneroo7 in 100
Balcatta6 in 100
Mount Lawley6 in 100
Bicton5 in 100
Bunbury5 in 100
Albany4 in 100

Download the simulations generated by the model here.

Update (18/Feb/2021): I’ve fixed a bug which resulted in the Others vote being too high and the “big five” (Labor, Liberal, National, Green, One Nation) vote being too low (as compared to what our vote models say they “should” be). While this doesn’t affect the topline forecast by much (the changes are under 0.1%), there are fairly significant seat-by-seat shifts, especially in seats with unusually strong Others candidates.

I opted not to redo the forecast for dates prior to 18/Feb, as I felt that that’s kind of cheating in a way (if I messed up the forecast, that’s on me and the change in forecast graph should reflect that). Just be aware that the model prior to 18/Feb and the model after 18/Feb are slightly different in interpreting the change-in-forecast graph.

I’ve also added the final-two candidates to the simulations output above.


Update (28/Feb/2021): Added the median district table above. I’ve also opted to round all probabilities written above (e.g. “7.6 in 100”) to the nearest whole number (e.g. “8 in 100”); reading through Nate Silver’s series on The Story of 2016 and Andrew Gelman’s article on incentives for forecasters has convinced me that having the decimal places likely provides a false sense of precision to the forecast.

Also added some info in our simulations download on how many electorates we expect to “call correctly” – i.e. the number of electorates where the party we estimate has the highest chance of winning actually wins. For example, if we estimate Labor has a 40% chance of winning, the Liberals 35% and the Nationals 25%, and Labor wins the seat, that would be a “correct call” even though the Liberals/Nationals had a combined 60% chance of winning. I personally don’t think that this is a very good way of looking at a probabilistic forecast, but it tends to be how the public and the media judge a forecast, so I’ve included it for informational purposes.

In addition, I’ve also added what I believe to be a better (if harder to interpret) way of judging forecasts to our simulations package – the Brier score our forecast would get in each simulation. This Brier score is calculated solely off the 2pp in each district – i.e. it uses Meridiem’s forecast of which major (Labor vs Lib/Nat) is likely to win the 2pp in each district instead of using the forecast of which party is likely to win the district overall. The reason for this is because the Brier score isn’t very good at judging forecasts for events which happen very rarely (e.g. a minor party winning a randomly selected district), so calculating a multi-category Brier score which includes the minor parties would make our Brier score look a lot better than it really is.

(if you don’t understand any of that, that’s alright – I’ll be writing up a piece about judging probabilistic forecasts like ours, due approximately the week before the election)

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