Over the last 30 years or so, probably longer, the size and distribution of the informal vote that we see at every election can be largely explained by a handful of variables -- with the election on Saturday being no exception.
The first of these variables is ballot length -- the number of candidates we see standing in each electorate. The mechanics of this one are fairly obvious, where a given voter is more likely to make a mistake filling out the ballot when there are say, 13 candidates on the ballot paper then they would if there were only, say, three candidates on the ballot paper. The more candidates there are on the ballot, the more human input there has to be to fill in the ballot, meaning the number of mistakes increases. On Saturday’s election, the number of candidates on ballots ranged from a lowly three through to a high of 11.