My forecast for the Australian 2019 federal election is a probable narrow ALP win with 75 to 82 seats in the House of Representatives. However, a range of other options are very much in play. An 80% prediction interval for the ALP is 70 to 87 seats, with the election to be held on 18 May 2019.
The Liberal-National Coalition is likely to end up with between 59 and 76 seats, but an outright win is still a possibility, and a win with the help of independents or minor parties more so. The Greens are most likely to end up retaining their single seat, but have a fair chance of picking up a second. In total, parties other than the ALP and Coalition are likely to end up with four to seven seats.
|Party||Lower estimate||Upper estimate|
The full distributions are shown in the chart below. This is based on 10,000 division-level simulations that take into account uncertainty in the overall swing as well as random division-level variation in swings. Division level swings are assumed to be normally distributed with a standard deviation of 3.2 percentage points, which is a fairly stable figure historically.
For contests other than ALP versus Lib/Nat coalition, trends in the two-party preferred vote are ignored and the simulation is done as straight out random variation from the most recent result. In my view, there isn't enough publicly-available (or probably, any) polling to do anything else reliable in these situations.
For seats where the main contest is ALP versus the Coalition, the simulations above include a nation-wide swing that is forecast with a Bayesian state space model drawing on all publicly available national level polls, taking into account the house effects which show the systematic under- or over-estimates of the polling firms of two party preferred vote in previous elections. That model is summarised in this next chart:
The "house effects" of the different polling firms can be seen below. We can't be sure of the exact average bias of each firm, so we have a distribution of likely results. As an example interpretation of this chart, Roy Morgan is estimated to over-estimate the two-party preferred vote for the ALP by somewhere between 1 and 2.5 percentage points.
This page will be updated at least weekly as new polls become available. Code for all this modelling is available on GitHub.