BludgerTrack 2016 methodology

BludgerTrack 2016 is an aggregate of all published national opinion polls: Newspoll, Nielsen, Galaxy, Morgan, Essential Research and ReachTEL. Each poll is adjusted to account for observed biases and weighted according to sample size and past reliability. Local regression analyis (LOESS) is used to plot trendlines through the available data and determine current results based on the modelled results for the most recent available point in time. State-level polling is used to track each state’s deviation from the national result, which is in turn used to produce vote and seat projections for each state.

Bias adjustment

The preferred method for determining bias is to determine an average error based on pollsters’ final pre-election polls against the ensuing election results. However, because federal elections are infrequent and only a few pollsters conduct state election polling, in most cases the number of observations available within an acceptably recent time frame is prohibitively small. The exceptions are Newspoll, Nielsen and Galaxy, for which measures have been derived based on federal and state polling since the start of 2010. For pollsters which deviate from the model in a generally consistent fashion, as has been determined to be the case for Morgan and ReachTEL, the deviance from the model over the full course of the term is measured and controlled for. Since the deviation for Essential Research appears to be variable, trend measures of its deviations are recalculated every week and applied to the published results.

Accuracy weighting

Pollsters are also weighted according to their historical accuracy, so that polls with better records have a greater influence on the total result. In the case of Newspoll, Nielsen and Galaxy, this has been determined by measuring the historical accuracy of pre-election polls going back to the start of 2010. For the newer poll series, results over the current term have been compared with the aggregated BludgerTrack results at the time of the poll’s publication.

For each comparison, the accuracy of the primary vote results for Labor, the Coalition and the Greens are measured using a normal distribution function, resulting in a percentage figure representing the proportion of poll results that would be expected to be less accurate when taking into account the poll’s sample size and theoretical margin of error. A composite of the three results is then produced with the result for each party weighted according to its share of the vote. The final accuracy measure is derived by indexing each pollster’s composite result against Newspoll’s. This means Newspoll’s accuracy measure is always 1, with other pollsters rated either higher or lower depending on whether they are found to be more or less accurate.

It should be noted that polls with larger samples tend to have lower accuracy ratings, as their lower theoretical margins of error mean they are held to a higher standard in the accuracy weighting calculations. This is especially notable in the case of the large-sample Morgan and ReachTEL polls.

National and state result aggregation

A total weighting score for a given poll is determined by multiplying its sample size by its accuracy measure. The trend charts are then generated from the bias-adjusted poll results using LOESS, with the modelled results for the most recent available date used to determine the headline BludgerTrack results. For the states and territories, deviations from the national result are determined using all available state-level data, with LOESS used to calculate a trend measure for the most recent available point in time. This includes both published and, from those pollsters kind enough to provide me with it, unpublished data. The deviation measurements are then applied to the national BludgerTrack totals to determine state voting intention results.

Electorate two-party projections

Two-party results are projected for each electorate based on the state swing figures, subject for adjustment where a seat is being vacated by a sitting member, or where a member first elected at the previous election can expect to enjoy a “sophomore surge”. The effect of these factors has been determined using regression modelling based on historical election results going back to 1993, which takes into account their tendency to have a greater impact in non-metropolitan electorates.

Seat totals

Seat totals for each state are determined by adding together win probabilities for each electorate and rounding to the nearest whole number, as distinct from the blunter “Mackerras” technique of assuming uniform swings and marking off seats with margins lower than the swing. Win probabilities for a given party are determined by assuming a normally distributed swing and calculating the likelihood of a two-party preferred result above 50%. Historical observation has been used to determine a relationship of y=0.131x+0.019 between the standard deviation (y) of swings within a given state and the state’s overall swing (x). This works out to a 1.9% standard deviation in the event of no swing, increasing to 3.2% for a swing of 10%.

Independent and minor party seats

Since BludgerTrack is essentially a two-party model, complications are presented by the five electorates which were not won by the major parties in 2013 — Kennedy (Bob Katter, Independent, Queensland), Denison (Andrew Wilkie, Independent, Tasmania), Melbourne (Adam Bandt, Greens, Victoria), Fairfax (Clive Palmer, Palmer United, Queensland) and Indi (Cathy McGowan, Independent, Victoria). It will be presumed that the incumbents in these seats will be returned unless electorate-level data emerges to suggest otherwise.

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