There is near-universal agreement among electoral specialists that the crucial determinant of an electoral system's
ability to translate votes cast into seats won proportionally is the district magnitude, which is the number of members to be elected in each electoral district.
Under a system such as FPTP, AV, or the Two-Round System, there is a district magnitude of one; voters are electing a single representative. By contrast, all PR systems, some plurality/majority systems such as Block Vote and PBV, and some other systems such as Limited Vote and SNTV, require electoral districts which elect more than one member. Under any proportional system, the number of members to be chosen in each district determines, to a significant extent, how proportional the election results will be.
The systems which achieve the greatest degree of proportionality will use very large districts, because such districts are able to ensure that even very small parties are represented in the legislature. In smaller districts, the effective threshold is higher. For example, in a district in which there are only three members to be elected, a party must gain at least 25 per cent +1 of the vote to be assured of winning a seat. A party which has the support of only 10 per cent of the electorate would be unlikely to win a seat, and the votes of this party’s supporters could therefore be said to have been wasted. In a nine-seat district, by contrast, 10 per cent +1 of the vote would guarantee that a party wins at least one seat. The problem is that as districts are made larger—both in terms of the number of seats and often, as a consequence, in terms of their geographic size as well—so the linkage between an elected member and his or her constituency grows weaker.
This can have serious consequences in societies where local factors play a strong role in politics or where voters expect their member to maintain strong links with the electorate and act as their ‘delegate’ in the legislature.
Because of this, there has been a lively debate about the best district magnitude. Most scholars agree, as a general principle, that district magnitudes of between three and seven seats tend to work quite well, and it has been suggested that odd numbers such as three, five and seven work better in practice than even numbers, particularly in a two-party system. However, this is only a rough guide, and there are many situations in which a higher number may be both desirable and necessary to ensure satisfactory representation and proportionality. In many countries, the electoral districts follow pre-existing administrative divisions, perhaps state or provincial boundaries, which means that there may be wide variations in their size. However, this approach both eliminates the need to draw additional boundaries for elections and may make it possible to relate electoral districts to existing identified and accepted communities.
Numbers at the high and low ends of the spectrum tend to deliver more extreme results. At one end of the spectrum, a whole country can form one electoral district, which normally means that the number of votes needed for election is extremely low and even very small parties can gain election. In Israel, for example, the whole country forms one district of 120 members, which means that election results are highly proportional, but also means that parties with only small shares of the vote can gain representation and that the link between an elected member and any geographical area is extremely weak.
At the other end of the spectrum, PR systems can be applied to situations in which there is a district magnitude of only two. For example, a system of List PR is applied to two-member districts in Chile. This delivers results which are quite disproportional, because no more than two parties can win representation in each district. This has tended to undermine the benefits of PR in terms of representation and legitimacy.
These examples, from the opposite ends of the spectrum, both serve to underline the crucial importance of district magnitude in any PR electoral system. It is arguably the single most important institutional choice when designing a PR system, and is also of crucial importance for a number of non-PR systems as well. The Single Non-Transferable Vote, for example tends to deliver moderately proportional results despite not being in essence a proportional formula, precisely because it is used in multi-member districts. Similarly, the Single Transferable Vote when applied to single-member districts becomes the Alternative Vote, which retains some of the advantages of STV but not its proportionality. In Party Block Vote and Block Vote systems, as district magnitude increases, proportionality is likely to decrease. To sum up, when designing an electoral system, district magnitude is in many ways the key factor in determining how the system will operate in practice, the strength of the link between voters and elected members, and the overall proportionality of election results.
On a related note, the party magnitude (the average number of successful candidates from the same party in the same electoral district) is an important factor in determining who will be elected. If only one candidate from a party is elected in a district, that candidate may well be male and a member of the majority ethnic or social groups in the district. If two or more are elected, balanced tickets may have more effect, making it likely that more women and more candidates from minorities will be successful. Larger districts (seven or more seats in size) and a relatively small number of parties will increase the party magnitude.
