Proportionate representation is defined such that the number of citizens in an area must be proportionate to the number of representatives for the same area. It is sometimes called Local Representation. It is one of many types of representation which can be considered in a Representative Government.
The degree of Proportionate Representation for each region is defined by the difference between the percent of seats obtained and the percent of the population (not voters) in that region. In a similar manner to Proportional Representation there is no agreed upon metric for combining these differences. However, since most systems have Balanced representation in that districts are defined by having equal population the exact calculation of Proportionate Representation. What is more common is the granularity of the Regional System, or the number or seats per region. Single member systems have the maximum possible granularity of Proportionate Representation if the population is divided evenly by population.
Original Term Use
In Canada, Proportionate Representation is defined and ensured in each of the constitutions. Specifically, it is ensured that the Provinces have Proportionate Representation of the provincial level in Section 52 and 42 of the constitution act of 1867 and 1982, respectively. These constitutions are the first known use of this term in this context. The concept was further codified in the Fair Representation Act in 2011. In Canada, this means that a regional system is required at least on the Provincial level in Canada so that the number of seats for each province is proportionate to the number of citizens in each province. This puts a fair amount of constraint on what electoral reform is possible without a constitutional amendment . Proportionate Representation is defined such that Balanced Representation is directly implied. Recall Balanced Representation is that each member represents a similar number of citizens. It is not guaranteed by Partisan Systems or Proportional Representation.
In Regional Systems
In a Regional system where members are elected to represent each region the issue is straight forward. If the population size is equal in each region the results are said to be proportionate. The issue then moves to a discussion of the granularity, ie how many representatives per region.
There are two ways that Proportionate Representation can be purposely altered. The first is an attempt to improve it. A good example of this is the election of leaders for the Conservative party of Canada. This is held through a general election by all party members using Instant-runoff voting. Since it is known that the number of party members in each riding is not equal it is not expected that the the outcome would be different if each voter participated in that riding. To improve proportionate representation the ballots from each region are weighted according to the number of party voters in the region.
In Partisan systems
In a partisan system several members of parliament are elected to represent parties and no region. In a Party List system Proportionate representation is at is worst because all regions have no representation. A Mixed Member Proportional system with half the members elected for regions and the other half for parties would represent the mid point between the optimal with a single member regional system and the worst with Party List. Additionally, such a systems would also have half the granularity of the Proportionate Representation