Proportionality
The outcome of voting under STV is proportional within a single election to the collective preference of voters, assuming voters have ranked their real preferences and vote along strict party lines. However, due to other voting mechanisms usually used in conjunction with STV, such as a district or constituency system, an election using STV may not guarantee proportionality across all districts put together. Differential turnout across districts, for example, may alter the impact of individual votes in different constituencies, and when combined with rounding error associated with a finite number of winners in each constituency the election as a whole may throw up anomalous results.
For example, the 1981 election in Malta resulted in the Labour Party winning a majority of seats despite the Nationalist Party winning 51% of the first-preference vote. Controversy over the election ultimately resulted in a constitutional crisis, leading to an amendment adjusting the voting system to allow for the possibility of bonus seats and making the Maltese voting system more similar to an open-list PR system; STV alone would also have given the second most popular party a parliamentary majority in the 1987, 1996 and 2008 Maltese elections. This kind of difference due to rounding error can occur with any PR system used at a district level, although greater rounding error occurs with smaller districts and there is a tendency for STV elections to use smaller districts when compared with PR elections employing party lists.
Similarly to differences in voter turnout, instances of malapportionment across districts can also cause disproportionate results for the legislature as a whole. In STV elections to the Australian Senate, states with vastly different populations have the same number of seats, and so while the results for individual states are proportional, the nationwide result is not, giving greater voting power to individual voters in less populated states. This is because the senate's role is to act as representatives of their states rather than representatives of the people. Thus the number of Senators does not take into account the population of the state or territory and thus by nature it is not intended to be proportional across the nation. However the system is intended to be proportional at a state level as the make up of that states senators will be comparable to the percentage that they gain of the state vote. This system is intended to remove potential geographical discimination that can occur due to Australia's large size. If the system was proportional across the nation then a party could focus on the eastern states that have 77% of the population. Thus if allocated proportionately the eastern states would gain a disproportionate amount of political attention and thus government funding due to the concentration of voting power.
By contrast, the New South Wales Legislative Council avoids the use of districts entirely, electing all 21 members using a single, statewide constituency and guaranteeing results that are proportional to the final allocation of preferences. In Victoria, Australia, legislation is in place to ensure that each electorate contains the same number of constituents. Problems occur when the number of candidates to be elected is different in each electorate, as the percentage of voters required to elect a candidate varies, whilst the number of voters remain the same. The percentage breakdown varies considerably, adding to the argument that the number of voters and the percentage required for election should be the same across all electorates. Under Victorian legislation an electoral review is required should the number of electors in each electorate fall outside the nominated tolerance level. Municipal Electoral reviews are undertaken every second term of office.
STV differs from other PR systems in that it allows the voter to decide the issues that should matter for the proportionality. In most PR systems the voter can only influence a single aspect - party representation. STV allows the voter to choose other criteria that can be used to create the proportionality: gender, ethnicity, age, place of residence or character as examples. A perceived problem arises from the fact that voters and the political elite are often not in accord with what sort of proportionality should be achieved. Increased voter choice and control reduces the power and influence of the political elite to determine the political debate.
STV provides proportionality by transferring votes to minimise waste, and therefore also minimises the number of unrepresented/ disenfranchised voters. In this way STV provides Droop proportionality - an example STV election using the Droop quota method for 9 seats and with no exhausted preferences would guarantee representation to every distinct group of 10% of the voters, with at most only 10% of the vote being wasted as unneeded excess (in most cases it would be much less). Unlike other proportional representation methods employing party lists, voters in STV do not explicitly state their preferred political party (with the exception where above-the-line voting systems are in place); this in turn can create some difficulty when attempting to analyze how an STV election's results compare with the nationwide partisan makeup. One common method of estimating the party identification of voters is to assume their top-preference on their ballot represents a candidate from their preferred party, however this method of estimation is made more complicated by the possibility of independent candidates and of cross-party voting. However valid comparisons have and can still be made if sufficient data and information is available. In Victoria, Australia it is possible to make a direct comparison between the Australian Senate election and the Victorian Upper House elections although individual circumstance will always exist voting patterns have shown that most voters stick to their chosen party within a limited percentage range based on local issues and circumstances. The main advantage in Victoria's case is that both systems are similar in design with one being a subset of the other. Victoria held its first multi-member proportional representation election in November 2006 for the Legislative Council
Read more about this topic: Issues Affecting The Single Transferable Vote