Optional preferential voting

Possibilities are: Ranked-voting systems typically use a ballot paper in which the voter is required to write numbers 1, 2, 3, etc.

Some OPV jurisdictions permit a ballot expressing a single preference to use some other mark than the digit '1', such as a cross or tick-mark, opposite the preferred candidate's name, on the basis that the voter's intention is clear; other do not, arguing for example that an 'X' might be an expression of dislike.

On the other hand, FPV increases the risk of invalid ballots: the more numbers a voter is required to mark, the greater the opportunity for mistakes, by repeating or skipping numbers or skipping candidates.

The Australian election systems used in almost all the state lower houses (all except Tasmania and New South Wales) and in the ACT mandate FPV but they reduce the number of informal (invalid) votes by adding group voting tickets "above the line" on ballot papers.

[1] The terms OPV and FPV are used mainly in Australia, in relation to elections at state and territory and Commonwealth (federal) level, which use proportional representation (Single transferable voting/STV) or preferential voting (IRV).

[7] Around 1999 Melbourne's Albert Langer engaged in a campaign to assist voters to mark fewer than all the candidates but still have their vote not deemed informal and rejected.

Since in the past a single number '1' above the line was valid, that is still a formal vote even though voters are encouraged to rank six candidates.