Pros
- Politicians need a wider base of support than First Past the Post
- SV stops candidates winning on low levels of support, but doesn't need them to get half the vote as a first preference (ERS)
Cons
- It requires guesswork - voters need to guess who will get into the second round so they can maximise the chance of their second vote counting. This is worse in a multi-party context
- If your second choice candidate does not make it to the "run-off" your second preference is wasted
- Like FPTP SV was designed for two-party politics, it is not multi-party system
- As SV only lets voters express two choices, in a multi-party situation many of the voters who cast a first choice for a candidate that doesn’t make the top two are also likely to cast their second choice for a candidate that is not in the top two, and so their vote will be wasted
- In almost two decades of SV elections for the Mayor of London, only in 2016 has a mayor won with a combined first and second preference vote which is more than 50% of the total votes cast, as many second preference votes do not count
- Little room for small parties to advance
- "wrong winner" elections can happen as with FPTP e.g. Australia 1954 (AV) but this can happen under many voting systems
