Time Trends and Primary Votes
Posted by Possum Comitatus on June 26, 2007
If we look at the Newspoll primary vote level of the three political blocks; the ALP, the Coalition and the Minor Party+Undecided group and apply a quadratic time trend to that vote of the type:
We get some very interesting results.
The above graph measures the time trend for the Minors+Undecideds group. The size of the primary vote can be read from the right hand side of these graphs, on the left hand side (the blue line), the residuals from the time trend regression can be read. Those residuals represent how far away from the time trend the primary vote is at any given time.
What stands out is the growing size of the Minors primary vote. Where is it coming from?
Well let’s do the ALP quadratic time trend next:
The minors were growing at the expense of the ALP up until the 1998 election. After that election, the ALP has itself experienced a growing primary vote trend.
No guesses what the Coalitions time trend looks like:
Up until the early 1990’s the Coalition was pulling in votes from the ALP as well, but since then their primary vote has been slowly declining, leaking to both the Minors and the ALP.
To sum up:
The Minor primary vote is growing, but at a decreasing rate and has been doing so for 20 years.
The ALP vote is growing, at an increasing rate and has been doing so since the end of 1998.
The Coalition vote is declining at an increasing rate, and has been doing so since 1994.
Those trends hold over the long term, but what about the short term, say just since 1996?
Well this time well use a cubic time trend of the form:
….which is the time trend that best fits the data for this shorter term.
So while these shorter term trends dominate the data, they are all actually consistent with the longer term quadratic trends looked at earlier. These cubic trends explain some of the shorter term volatility that occurs around the longer term quadratic trends.