In voting, a ballot is considered to be spoilt, void, null or informal if it is regarded by the election authorities to be invalid and thus not included in the tally during vote counting. This may be done accidentally or deliberately.
In the United States, it is common to distinguish between three classes of ballots:
These distinctions arise largely because of the common practice in the United States of voting on a large number of different issues on a single ballot.
Ways of spoiling a ballot include:
Ballot design and voter instruction are intended to minimise accidental spoiling of votes. Some election officials have discretion to include ballots where the strict criteria for acceptability are not met but the voter's intention is clear. More complicated electoral systems may be more prone to errors. Group voting tickets were introduced in Australia owing to the high number of informal votes cast in single transferable vote elections.
Spoilt votes may be the result of a deliberate act by the voter; some proportion are likely to be protest votes, especially in systems where voting is compulsory. Intentionally spoiling someone else's ballot before or any ballot during tabulation is a method of election fraud. In Australia, inciting others to vote informally is illegal, though informal voting is not.
Paper-based voting systems are thought to be more susceptible to unintentional errors that spoil ballots; some paper-based voting systems and most DRE voting machines can notify voters of undervotes and overvotes.
The validity of the election may be questioned if there is an unusually high proportion of spoilt votes, however, in countries such as the UK where a spoilt ballot paper counts towards the voter turnout, some voters will deliberately spoil their ballot paper to show disapproval of the candidates available whilst still taking part in the electoral process. In theory, a UK election could have a 100% turnout with no votes cast for any of the candidates if every registered voter were to spoil their paper, although this is highly improbable.