In every election, some votes cast are informal or invalid. Several things can render a ballot paper informal and, in Australia, rules for formality differ between state and federal elections, and between Senate and House of Representatives ballots. In the mandatory full preferential voting system used in federal elections for the House of Representatives, a ballot will be considered informal if:
These ballots do not count towards any candidate, and are counted separately. For analytical purposes, the AEC sorts and categorises informal ballot papers into several categories according to the reason for their informality.
The level of informal voting (the 'informality' of an election) is influenced by many factors. In Australia, these include include compulsory voting, differences between the voting systems of the states and that of the Commonwealth, and sociological factors. The AEC published a paper in 2001 which fully examined these factors.
This research paper profiles informality in the 2004 election for the House of Representatives, and attempts to explain increases in informal voting since the 2001 election using 2004 data. The paper uses a multiple regression model to examine informality levels and the change in the number of candidates for all 150 Divisions. The findings of the paper emphasize that a change in the number of candidates contributes to variations in informality. In 2004 there were an average of 7.27 candidates per division, an increase from 6.92 percent in 2001. Informality increased nationally from 4.80 percent of the total of ballots in 2001 to 5.18 percent in 2004.
The paper also examines correlations between socioeconomic indices and categories of informality.
The paper concludes that the main cause of the rise in informality from 2001 to 2004 is the increase in ballots declared informal because no preferences are stated and scribbles, profanities or other marks are written on the ballot. This 'Marks or scribbles' category has also increased as a proportion of all informal ballots (from a national average of 6.39% in 2001 to 14.27% in 2004). Had this category of informality remained static in 2004, overall informality would not have changed significantly from the 2001 House of Representatives election.