I usually explain STV this way (there may be no perfect way):
under STV, there are multiple members elected in a district. each voter has one vote so no one group can take all the seats.
(maybe have to explain how under block voting one group with only say 35 perent of the voters can take all the seats;
how under FPTP often members are elected with just minority of the vote - that alone blows people's minds after all the majority myths they have been fed all their lives)
That ensures mixed representation, so parties get about the number of seats that their vote share makes their due.
the vote cast by the voter can show back-up preferences. that allows transfers if necessary. if the vote is cast for someone who is unelectable, the vote may be transferred to a diff cand.
or if vote cast for someone who is popular, the surplus votes are transferred to a diff cand.
That ensures that each elected member is elected with same number of votes, and so that means that each party gets its due share of seats.
(I might say, if person still listening, that members might be elected at end just by being more popular, not by "quota," but still STV's fairness is much more fair than under the FPTP system where one member in one district might be elected with 4000 votes while another in another district might be elected with 40,000.)
(and if they ask how surplus votes are determined, explain idea of quota. the Droop is tough nut to explain - Hare quota is easier to explain so saying...)
Quota is approx the number of votes divided by number of seats to fill. That determines the number of surplus votes and prevents waste due to one candidate taking more than actually required to be elected. Well they take the votes but then the surplus votes are transferred away from them, leaving them just with quota. The surplus votes then go elsewhere where they might be used to elect someone else.
no, each vote is only used once to elect someone. that is the S in STV.
the back-up preference are like sending someone to the store, telling them if you can't get chocolate ice cream, get strawberry, if not, then get vanilla, but no sherbet. they are instructions to the election official of what to do so you don't come home empty-handed.
but if there is chocolate ice cream, then you get your first wish.
well, a city-wide district is a natural way to district. the mayor is elected that way. How about 12 councillors or r(R)epresentatives elected to cover same area? With each one elected by equal sized group of voters, either spread across the city or concentrated in any one part of the city. all is fair. Right now city is divided into arbitrary provincial, city and federal districts not making sense on the ground. a candidate might lose in one ward with more votes than the number that elected a candidate in a diff district. not fair.
by electing multiple members in one district, there are direct comparison between more candidates and thus more fairness. right now the micro-districts used in FPTP produce waste when most of the little bits of votes in each district just go into the trash. and only one member is said to represent all in the district when all sorts of people live in the district and in fact maybe a majority voted against the elected member.
(visa vis city elections) no you don't need parties for STV, STV does not have to use parties. Candidates in government election will have party labels but votes are transferred as voter marks, not necessarilly along party lines. STV is candidate based, not party based.
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anyway something like that
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Droop quota is used today in STV.
(valid votes cast / (seats plus 1) ) plus 1
5 seat district 12,000 valid votes quota is 2001
possible for five candidate to get quota (and be elected) while some other candidates are not eliminated - the vote of the other candidates are not transferred (you could say they go into trash).
But often in optional-preferential STV, there are so many exhausted (non-transferable) votes that final seat or seats are filled at the point in the process when the field of candidates is thinned to number of remaining open seats, irrespective of quota. (there are no candidates who are not either elected or elected.)
or sometimes there is just one candidate who is not either elected or elected. (the last one who is declared defeated, without subsequent vote transfers)
the filling of final seat by candidate who has only partial quota may be caused by non-transferable votes accumulating to point where no longer possible for last seat tobe filled by candidate with quota - there is just not a quota of votes out there still in play.
rule is process ends when seats are filled or when the field of candidates is thinned to number of remaining open seats.
the successive vote counts end when (if) field of candidates is thinned to one more than number of remaining open seats, then the least-popular cand is declared defeated and his/her votes not transferred, the remaining candidates being declared elected to fill the seats. (This happens after last recorded vote count, such as say
Count 23 elimination leaves two candidates remaining (D and E) and one open seat. Count 23 is last count because then (not shown in detail) after count 23, the least-popular cand (say E) is declared defeated and the most-popular candidate (D) is declared elected.)
for information on how STV works, quota etc. see Wiki "Single transferable voting
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