What do we want to do if we wish to measure voters’ ideology (or issue preferences, for those who don’t like that word)?
Unfortunately, here in Australia the we don’t really have survey data available which asks a large number of consistent questions over time, which would allow us to compare voters of, say, the 1960s, with the voters of today.
Additionally, there’s also the question of how useful is survey data for measuring voter attitudes? Based on his analysis of survey repsonses, Philip Converse didn’t think most voters’ had a comprehensive ideological framework within which they organised their political preferences; although later research has indicated he might have been a little harsh, with measurement error obscuring the real political preferences of the voters he was examining.
One way of dealing with the lack of consistent questions over time and issues of measurement error, is to model the latent ideological traits using graded Item Response Theory (IRT) models.
Although much of the original work on IRT estimation was conducted by education scholars to measure latent traits such as intelligence or personality, more recently these methods have been developed to measure the latent ideological attitudes of political actors, such as justices of the United States Supreme Court (Bafumi et al. 2005) and voters (Treier and Jackman 2002, Treier and Hillygus 2009). The model I look at here was fit to 34 issue items from a combined file of all 12 Australian National Political Attitudes Survey (ANPAS( and Australia Election Study AES) surveys using the mirt package (Chalmers 2012) in R (R Core Team 2013). These surveys provide information on the policy preferences, voting behaviour and demographic backgrounds of 24,393 respondents sampled from 1967 to 2010.
I measure the latent issue preferences of voters on a left-right spectrum (standardised to have a mean of 0 and variance of 1) and two dimensions (effectively an economic and social issue dimension), with the results for the economic dimension shown below.
Each plot shows the economic issue preferences of the median voter of five of the more significant parties to contest Australian federal elections over the 44 years from 1967-2010, compared to the actual median voter for each survey. The higher scored preference, the further to the right each party’s median voter is. I’ve plotted a linear trend line for each party to make patterns clear.
The results are not particularly surprising. Greens and Labor voters are left of centre. Coalition voters are right of centre, with One Nation slightly more centrist on economic issues. What is interesting is that the Democrats start at the centre (not surprising) and then shift well to the left over a 20 year period. Also interesting, the median Coalition and Labor voter has moved to the right compared to the electorate median over this time (likely as both lost their more leftwing voters to the Democrats and then Greens).