Possum: what size the ALP vote hit from closing rolls early?

When Julia Gillard announced August 21 as the election date rather than August 28, one of the consequences was that the electoral roll closed a week earlier than it ordinarily could have — providing very little time for younger voters to get themselves on the roll. As we know, younger voters — especially the 18-24s — have historically voted for the ALP in two-party preferred terms at a much higher rate than the broader population, so today we’ll have a squiz at the size of the sacrifice in national two party preferred vote share Labor could expect to experience as a result of smaller numbers of their most favourable age cohort being enrolled than would have been the case if the election was called for a week later.

In April of this year, the AEC estimated that there were about 430,000 Australians aged 18-24 who were not enrolled to vote. At the end of June, the latest period we have enrolment data for, there were 1,450,267 18-24s on the roll — meaning that about 77% of that cohort was enrolled. There was also 12,451,573 people aged 25 and over on the roll, giving us a total electoral roll of 13,961,671.

We also know that 18-24s in the polling have been supporting Labor at a rate of 54%-62% for the past couple of years. At the 2007 election they actually voted for Labor somewhere about the 64% mark.

To estimate the consequences of 18-24 enrolment size on the ALP national two-party preferred vote share, if we assume that 18-24s vote at 58% Labor (the mid point of the polling) and hold all other age cohorts constant at 50% ALP vote — we can look at how the ALP two-party preferred vote increases as those 430,000-odd young voters are added to the roll in 1% increments.

It doesn’t really matter what we hold the voting level of people 25 and older at, as that only changes the level from which the vote estimates change, not the actual rate of change in TPP that the 18-24s create — which is what we’re actually interested in here. We’ll also use 95% as the maximum enrolment level we could expect to see among the 18-24s, as Peter Brent (of Mumble fame) and Simon Jackman suggested that 95% was about the level of enrolment we saw for the 25 years and older cohort in the 2007 election.