The second in a five-part series looking at the different voting systems.

How does the single transferable vote (STV) system work?

Take a deep breath. This is the most complex of the four alternatives to MMP. The country is split into electorates which each return (usually) three but possibly as many as seven MPs.

Voters rank the candidates in their electorate in order of preference (1, 2, 3 and so on). To win, a candidate must receive a fixed quota of votes which is determined by a mathematical formula.

If some candidates reach the quota during the counting of first preferences, they are elected. The first preferences received by those candidates above the quota are then redistributed according to the second preferences of those surplus votes.

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Still with us? If there are still vacancies to fill after the distribution of surplus first preferences, the lowest polling candidate is eliminated and all of his or her votes are redistributed in line with the second preferences of those who voted for that candidate.

But wait, there's more. If no candidate is elected after the counting of first preferences, then the lowest polling candidate is eliminated and his or her votes are redistributed in line with second preferences. The process is repeated until all the electorate's seats are filled.

Where is STV used?

Ireland, Malta, Tasmania, the Australian Capital Territory and Australia's federal upper house, the Senate. STV is used for the election of New Zealand's district health boards. It is also used by six New Zealand territorial local authorities including the Kaipara District Council and the Wellington and Dunedin city councils.

What are the advantages of STV?

Despite its complexity, STV ticks a number of boxes. Depending on its design, STV is broadly proportional. Yet, it maintains a direct link in terms of accountability between MP and voters, who effectively have a greater say over which MPs get elected from which party, thereby reducing the power of party bosses.

For example, in a five-member electorate, a party will put up at least three candidates. They might represent different factions within their party. Voters can also distribute their preferences to candidates from different parties.

There are therefore fewer "wasted votes" because most voters' preferences are likely to have had an influence in the election of at least one MP from a multi-member seat.

Voters have a choice of MPs to whom they can take any problems.

STV handicaps candidates and parties with extreme views. While they may secure a slice of first preferences, they are unlikely to win sufficient second preferences from mainstream voters to get elected. All this makes STV the favourite option of experts on voting systems.

What are the disadvantages of STV?

It would be goodbye to a great New Zealand tradition - sitting in front of the TV on election night waiting to see who will be running the country for the next three years.

Computers might be able to forecast the final results. But the official vote count might take some days to reach a final verdict.

A far more significant disadvantage is that the intense competition for votes within an electorate can lead to MPs doing little else but constituency work rather than concentrating on national issues.

This "clientelism" can even see MPs refusing to back unpopular measures until they are sure constituency rivals from their own party are doing likewise.

In Ireland, this behaviour is seen as a major drawback of STV, although some political scientists argue the behaviour is more down to the Irish political culture than the mechanics of the voting system.

The establishment of multi-member electorates would mean some would be very large in geographic area.

That might pose special problems in sparsely populated regions like the West Coast. As with all types of preference voting, there is a risk of "donkey voting", that is, apathetic voters ranking candidates in indiscriminate fashion without thinking - especially if voters have little knowledge of the candidates or cannot be bothered trying to understand how STV works.