In politics, the two-party-preferred vote (2PP), or two-candidate-preferred vote (2CP),[1] in an election or opinion poll uses preferential (instant-runoff) voting to express the electoral result after the distribution of preferences. At a seat level, the candidate with the lowest vote is eliminated and their preferences are distributed, which is repeated until only two candidates remain. Whilst every seat has a 2CP result, seats where the major parties have come first and second are commonly referred to as having a 2PP result. In seats where the major parties do not come first and second, differing 2PP and 2CP results are returned. At the 2010 Australian federal election, only eight of 150 seats[2] returned differing 2PP and 2CP figures, indicating a considerable two-party system.
To obtain the two-party-preferred result, it may be necessary to continue distributing preferences beyond the point at which the winner of the vote is known. For example, in an election with four or more candidates, it is possible that after the first round(s) of preference distribution the result could be:
Candidate A has more than 50% of the votes, so will be declared the winner of the election. However to determine the two-party-preferred result, it is necessary to distribute candidate C's votes between A and B. If 90% of C's preferences (i.e. 18% of the total count) are distributed to B, and the remainder (2% of the total) to A, the resulting two-party-preferred count would be:
A's two-party-preferred lead is only 6% – much lower than the lead of 22% at which A was declared the winner.
The two-party-preferred result is useful because it shows the overall level of support for each of the main political groups in a two-party system. In the above example, only slightly more than 3% of the voters need to change their primary votes or their preferences from A to B for B to win.
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The Australian Electoral Commission (AEC) uses the TCP vote count as a quick way to determine the results of an election in a seat.[1] In normal run-off voting, there is a series of counts, and at each count the candidate with the least votes is eliminated and their votes redistributed to the remaining candidates according to preferences. With TCP counting while election counting progresses, the two candidates with the highest primary vote totals are determined, and the votes of all other candidates are distributed via preferences to these two candidates immediately.
Before the election, the AEC will make a prediction (based on results at previous elections) at the two candidates so that on election night the TCP count for each seat can be quickly determined. In some seats the primary count will reveal that the prediction was incorrect, and there is a recount using the correct two candidates.
It is possible that a TCP count may yield a different result from true run-off voting. To guard against this, when considering primary votes, a TCP count is used to determine only the result of the election where the sum of all votes for candidates other than the two candidates is less than the number of primary votes received by the candidate with the second-highest primary votes total.[3] If this situation is not the case, a full instant run-off count is used instead.
In Australian elections, the two-party-preferred count recounts every vote as either a vote for the Coalition or the ALP, according to which party receives the earlier preference by the voter. This calculation is done by convention, as these two parties are the most likely to form government, and is used as an indication of the overall result. However, the TPP count has no bearing on the actual outcome of the election, and occasionally a party will form government even though it has received less than 50% of the TPP count.[1]
Australian opinion polls survey voter intention, generally through telephone surveys. Two-party-preferred (2PP) figures are calculated based on preference flows at the preceding election.[4] These figures are intended to estimate the TPP result at the next election. Sample sizes are too small to estimate results in individual seats, so only national 2PP figures can be estimated.