Editing Single transferable vote (section)

===Vote transfers and quota===
{{See also|Comparison of the Hare and Droop quotas}}
STV systems primarily differ in how they transfer surplus votes and in the size of the quota. For this reason, it has been suggested that STV can be considered a family of voting systems rather than a single system.

If fair results are to be produced and the number of candidates is fixed, a quota must be set such that any candidate who receives that many votes is elected. The quota, if used, must be set at a level where no more candidates can reach quota than there are seats to be filled. It cannot be so small that more candidates can be elected than the number of open seats, but the smaller it is, the fairer the result. There are several ways to specify quotas.

The Droop quota is the one most commonly used. It is generally considered to be the absolute lowest number that elects the correct number of candidates to fill the available seats, at least based on the original number of votes cast.

The [[Droop quota]] is given by the [[floor function]] formula:

<math display="block">\text{votes needed to win} = \left\lfloor \frac{\text{valid votes cast}}{\text{seats to fill}+1} \right\rfloor + 1</math>

where <math>\lfloor \ldots \rfloor</math> produces the integer less than or equal to its argument. The Droop quota is an extension of the [[majoritarian]] principle of a successful candidate having to get at least 50% + 1 in single-winner elections. No one else can get as much. In a three-seat contest, 25% plus 1 is the Droop quota because no more than three people can each have 25% of the vote + 1; using Droop means 10% of the vote + 1 is the quota in a nine-seat district because no more than nine people can each have 10% of the vote + 1, and so on.

Droop being relatively low means that the largest party, if it has the majority of votes, is likely to take the majority of the seats in a district. Additionally, a small party may have a chance to take a seat.
	
The [[Hare quota]] was used in the original proposals by [[Thomas Hare (political scientist)|Thomas Hare]].{{sfn|Lambert|Lakeman|1955|p=245}} It is larger than the Droop and sometimes ensures greater representation to less-popular parties within a district. But also, being larger than Droop, Hare presents more of an obstacle to small parties that hope to take just one seat. Being smaller than Hare, the Droop quota may give a seat to a small party that does not have the votes to take a seat under Hare.

Surplus votes cast for a winning candidate are sometimes transferred to the voter's next choice candidate, who is also preferred by the voter. (Any vote is only used once but may be allocated to different candidates along the way until it finds its final place.) Most first-count votes cast for a candidate who wins in the end are never transferred – just the surplus votes are transferred (unless all seats are already filled). Alternate preferences are only consulted if the candidate is unpopular or elected, and not always then. Votes lie where they are when the last seats are filled, so even under STV not all votes are used to elect someone.<ref name="auto">''A Report on Alberta Elections'' (1982)</ref>

There are variations in the conduct of transfers in different variations of STV, such as how to transfer surplus votes from winning candidates and whether to transfer votes to already-elected candidates.

It can happen that a vote is eligible to be transferred but cannot be because it bears no subsequent preference for any remaining candidate. In the case of transfers of surplus votes, an "exhausted" vote remains with the victorious candidates and only transferable votes (votes bearing a usable alternate preference) are used to determine the transfer of the surplus. If the number of transferable votes is less than the number of the surplus, no calculations are needed to make the transfer. Transfer of the transferable votes is done simply by reference to subsequent preference on the votes. Not all the surplus will be transferred if there are not enough transferable votes.

The STV systems in use in government elections today (such as in Malta and Ireland) do not allow votes to be transferred to candidates already elected. If the variation of STV used allows transfers to candidates already elected, when a candidate is eliminated and the next preference on the ballot shows preference for a candidate already elected, votes are transferred to the already victorious candidate, forming a new surplus. The new surplus votes for the victorious candidate (transferred from the eliminated candidate) are then transferred to the next preference of the victorious candidate, as happened with their initial surplus, but just using the recently transferred votes as guide. Vote transfers from the victorious candidate to a candidate who has been eliminated are impossible, and reference must be made to the next marked preference, if any. See {{section link||Seat filling by quota}} for details.

A different quota, one set lower than Droop, is sometimes workable. If fractional votes are used in an STV method, a quota smaller than the Droop quota may be used, where less than a whole number is added to votes/(seats plus 1).

The use of an even smaller quota is sometimes defended, although under such a quota, it is theoretically possible to have more candidates receive quota than the number of empty seats. Frank Britton, of the Election Ballot Services at the Electoral Reform Society, stated that the final "plus one" of the Droop quota is not needed; the quota he proposed was simply <math>(\text{valid votes cast}) / (\text{seats to fill}+1)</math>. The equivalent integer quota may be written:

<math display="block">\text{votes needed to win} = \left\lceil \frac{\text{valid votes cast}}{\rm \text{seats to fill}+1}\right\rceil </math>

So, the quota for one seat is 50 of 100 votes, not 51.{{sfn|Newland|1984}}

Even a low quota, such as the [[Imperiali quota]], is sometimes used. In any case, in most STV elections the appearance of non-transferable votes means that the quota could be lowered significantly below Droop during the counting of the vote with no danger of having too many achieve quota.

In STV, vote transfers are of two types{{Snd}}transfers of votes of eliminated candidates and transfers of surplus votes of elected candidates. The first type happens more often than the second type. Surplus votes are transferred only after a candidate is elected and then only if there are still open seats to be filled and if the transfers may affect the ranking of the remaining candidates, although rules vary from STV system to STV system.