Pros
- Wasted votes are minimised as both surplus votes and votes for losing candidates are redistributed.
- Proportional system - mostly works very well to match party seats and votes so long as the constituencies are large enough (more proportionate with larger constituencies).
- Voters more likely to have an MP for whom they have voted than under FPTP.
- High level of voter choice - voters can choose between candidates of the same party.
- Potential to express intra-party preferences (candidates from different traditions within a party available to choose on the same constituency list).
- Offers voters the chance to choose popular candidates rather than simply their preferred party (not choosing unpopular candidates that parties have put forward).
- Group of representatives reflect the diversity of opinions in the area (true for any PR system).
- Voters can vote for independent candidates without worrying about wasting their vote.
- It is a system where people use local knowledge to judge individual candidates at local level e.g. for local government.
- Extremist/populist parties’ candidates less likely to attract second or third preferences from supporters of other candidates.
Cons:
- The multi-seat nature of STV can lead to "excessive localism" according to Garret FitzGerald (ex-Ireland Taoiseach) because competing politicians of the same party act as glorified local ombudsmen rather than focusing on national policy.
- The Republic of Ireland is one of only two countries to use STV for their national parliament (the other is Malta), but both main Irish parties supported a referendum to change from STV (the Irish people voted “No”).
- Even with large multi-member constituencies, some smaller constituencies will not represent all parties (for example, a three- or four-seat constituency in a five-party system).
- The counting process in STV is complex and hard to explain to citizens, potentially endangering its legitimacy (but the process hasn’t been challenged, or its legitimacy endangered in practice).
- In Northern Ireland STV has not had as much impact as hoped in encouraging voting across sectarian dividing lines.
- Because it relies on transferred votes, it may promote bland “acceptable to all” policies
