BludgerTrack 2013 methodology
BludgerTrack 2013 is an aggregate of all published national opinion polls: Newspoll, Nielsen, Galaxy, Essential Research and Morgan. 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 measure each state’s deviation from the national trend, which is in turn used to produce vote and seat projections for each state.
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 two exceptions are Newspoll and Galaxy, for which the seven most recent case studies have been used, respectively going back to 2009 and 2007. A further exception in this respect is Morgan phone polls, for which I am content to use a performance average going back to 2007 despite the small number of case studies owing to its record of accuracy over the longer term. For other pollsters, bias measures are derived from a trend measure of their deviations from the BludgerTrack model, to be re-calculated monthly.
Pollsters are also weighted according to their historical accuracy, so that polls with better records have a greater influence on the total result. In most cases this has been determined by measuring the historical accuracy of their final pre-election polls. In these cases, the observations used are either the most recent ten results or, where fewer then ten are available, all results going back 10 years. For two newer poll series, Essential Research and Morgan’s multi-mode polling, poll results over the current term have been compared with the aggregated BludgerTrack results at the time of the poll’s publication.
For each pre-election poll, 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.
As of early April 2013, the accuracy measures and bias adjustments are as follows.
Weight ALP L-NP GRN Morgan phone 1.16 +0.1 -0.4 -1.7 Galaxy 1.14 +1.5 +0.4 -1.8 Newspoll 1.00 +1.5 -0.2 -0.8 Nielsen 0.59 +2.3 +0.5 -1.2 Essential 0.58 -0.2 -1.5 0.0 Morgan multi-mode 0.19 +1.6 +0.6 -1.3
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 Morgan multi-mode poll, for which sample sizes are around 3000.
National and state result aggregation
A total weighting score for a given poll is determined by multiplying its sample size by its accuracy measure. Pollsters which survey on a weekly basis, namely Essential and Morgan multi-mode, have their sample sizes halved to prevent them from dominating the overall result. 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 each mainland state, measurements of 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. The dominant datasets here are the quarterly Newspoll aggregations and the state breakdowns provided in each monthly Nielsen release. The deviation results are then applied to the national BludgerTrack totals to determine state voting intention results. For Tasmania, which is not covered by the Newspoll or Nielsen results, deviation measures will be determined on an ad hoc basis using whatever polling information is available. In the case of the four territory seats, the national swings are applied.
Electorate two-party projections
Two-party results are projected for each electorate based on the state swing figures, which are subject to two adjustments. Firstly, an electorate’s historical susceptibility to above or below average swings has been determined by averaging swings from the past five elections. The overall average is used as a baseline from which each electorate’s swing average is indexed, and an adjusted swing determined by multiplying the indexed figure by the state swing. Secondly, adjustments are made 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”. These adjustments, which have been based on historical observation, are as follows:
City Regional Rural Sophomore 1.0% 2.0% 2.0% Vacancy -0.9% -0.9% -1.2%
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 lower margins (though in practice it will rarely make much of a difference). Win probabilities are determined by assuming normally distributed swings and determining the likelihood of a result below 50% based on a seat’s two-party projection. Historical observation has been used to determine that a highly significant relationship of y=0.131x+0.019 exists between the standard deviation (y) of swings within a given state and the state’s overall swing (x). This calculates as 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
The five seats not currently held by major parties — New England (Tony Windsor, Independent, NSW), Lyne (Rob Oakeshott, Independent, NSW), Kennedy (Bob Katter, Independent, Queensland), Denison (Andrew Wilkie, Independent, Tasmania) and Melbourne (Adam Bandt, Greens, Victoria) — are going in the too-hard basket, although determinations will be made on an ad hoc basis as seat-level polling for these seats becomes available during the election year. For the time being, they are grouped on the seat total table as “not projected”.