One of the major arguments for First Past the Post (FPTP) is that it produces stable single party governments that have a mandate to carry out a clear set of policies on the basis of the support of the largest number of voters.
Proportional Representation (PR), in contrast, places a number of parties in Parliament, many of them small and this leads to unstable Coalition Government.
Small centrist parties with limited support from voters may be permanently in Government because they are always needed to form the governing coalition.
Against this, it is argued that FPTP cannot guarantee to produce a single party government that has the most support.
Differential turnout and the way that boundaries of the single member constituencies are drawn can lead to the party with most votes losing the election as happened to Labour in 1951 and may happen to the Conservatives in 2015.
A single party government is produced by FPTP if there are only two significant parties and, in multi-party systems where one of the two main parties receives many more votes than the other.
However, if the two main parties are close together as in February 1974 and 2010 in does not. If there are three or four parties with over 25% of the vote then small shifts in voter support can produce very different number of seats for the parties.
In Canada, when there were five parties with a significant vote, the Conservative Party went from a majority of seats in Parliament in 1988 to 2 seats in 1993.
Although PR will produce coalitions, these do not have to be unstable, as the current British coalition and most coalitions in Germany and Finland have shown.
Because FPTP is biased towards the larger parties, it tends to lead to centre-right and centre-left parties with moderate policies and if they move too far to the left or right then they rapidly lose seats.
Many conflicts of view are sorted out within the parties to produce an agreed policy.
PR produces a very fragmented system with small parties that only represent a particular viewpoint and extremist parties can be represented in Parliament.
Against this it can be argued that most European countries have a large centre-right and centre-left party under PR and in Germany, for example, these have taken part in moderate governments with the centrist Free Democrat party.
The adoption of a threshold in PR systems can keep out very small parties from Parliament or the Additional Member System, which elects some MPs FPTP and then adjusts the rest to produce a more proportional result, also tends to keep out very small parties.
Parliaments elected under FPTP do not reflect the range of views that there is in society and it is better to have extremist parties in Parliament, where they are forced to debate with other politicians, than keep them on the margins.
FPTP leads to wasted votes so that most voters do not take a part in deciding who represents them. In the general election of 2005 70% of voters did not get the MP they wanted.
Small numbers of voters in marginal constituencies decide an election.
In the 2010 general election, if 4288 voters had voted Conservative instead of Labour, the Conservative would have had a Parliamentary majority instead of needing to enter a Coalition; these voters represent 0.00009% of those who voted in the election.
PR systems mostly lead to a higher turnout because votes are not wasted. FPTP gives MPs to smaller parties whose votes are concentrated geographically but not to those with the same or a larger national vote if these votes are evenly spread.
FPTP means that voters can vote for an individual candidate to represent them and not just a party.
The closed party list system gives power to the central party rather than the voter to decide which candidates get elected and so more independent minded candidates, who the voters might like, are kept out.
It also preserves a link between an individual constituency and the MP so that there is geographical representation in Parliament, the voters identify more with their MP, and the MP is more likely to take up issues that are locally important.
Single Transferable Vote (STV) and open list PR systems allow voters to vote for an individual and STV gives voters a chance to rank candidates in order.
Under FPTP voters in Britain have resorted to tactical voting to keep out the party they least like but this forces them to guess who the two top candidates are likely to be.
The Additional Member system keeps the constituency link but still produces a more proportional system overall.
Under FPTP constituency parties may choose a safe candidate who they think will win whereas in closed list systems political parties can put arrange of candidates from different social groups on the list and can ensure balanced female and ethnic minority representation.
The FPTP system is straightforward and the voters understand it compared with PR systems.
In the Additional Member system in Scotland and Wales some voters thought they had to give their second vote to the same party and some thought they had to give it to a different party.
STV takes a long time to count – in the European elections in Northern Ireland, with not a high turnout, it took 26 hours.
Voters over time get to understand the new system. It is more important to get a system that is democratic than worry about taking a few more hours to count.
The Alternative Vote or Supplementary Vote systems can be used to overcome some of the problems of FPTP so that an MP has to get 50% of the votes, after preferences are counted, and so will have to persuade voters of other parties rather than just rely on the seat being safe for their party.
These systems can produce an overall result that is even less proportionate than FPTP.
The candidate that voters least dislike is likely to be elected rather than the candidate with most support.
