The highest averages method or divisor method is the name for a variety of ways to allocate seats proportionally for representative assemblies with party listvoting systems. It requires the number of votes for each party to be divided successively by a series of divisors. This produces a table of quotients, or averages, with a row for each divisor and a column for each party. The th seat is allocated to the party whose column contains the th largest entry in this table, up to the total number of seats available. An alternative to this method is the largest remainder method, which uses a minimum quota which can be calculated in a number of ways.
The most widely used is the D'Hondt formula, using the divisors 1, 2, 3, 4, etc. This system tends to give larger parties a slightly larger portion of seats than their portion of the electorate, and thus guarantees that a party with a majority of voters will get at least half of the seats.
The Webster/Sainte-Laguë method divides the number of votes for each party by the odd numbers and is sometimes considered more proportional than D'Hondt in terms of a comparison between a party's share of the total vote and its share of the seat allocation. This system can favour smaller parties over larger parties and so encourage splits. Dividing the votes numbers by 0.5, 1.5, 2.5, 3.5 etc. yields the same result. The Webster/Sainte-Laguë method is sometimes modified by increasing the first divisor to e.g. 1.4, to discourage very small parties gaining their first seat "too cheaply".
Imperiali
Another highest average method is called Imperiali. The divisors are 1, 1.5, 2, 2.5, 3, 3.5 and so on. It is designed to disfavor the smallest parties, akin to a "cutoff", and is used only in Belgian municipal elections. This method is not strictly proportional, if a perfectly proportional allocation exists, it is not guaranteed to find it.
In the Huntington–Hill method, the divisors are given by, which makes sense only if every party is guaranteed at least one seat: this effect can be achieved by disqualifying parties receiving fewer votes than a specified quota. This method is used for allotting seats in the US House of Representatives among the states.
Danish method
The Danish method is used in Danish elections to allocate each party's compensatory seats at the electoral province level to individual multi-member constituencies. It divides the number of votes received by a party in a multi-member constituency by the divisors growing by step equal to 3. Alternatively, dividing the votes numbers by 0.33, 1.33, 2.33, 3.33 etc. yields the same result. This system purposely attempts to allocate seats equally rather than proportionately.
Quota system
In addition to the procedure above, highest averages methods can be conceived of in a different way. For an election, a quota is calculated, usually the total number of votes cast divided by the number of seats to be allocated. Parties are then allocated seats by determining how many quotas they have won, by dividing their vote totals by the quota. Where a party wins a fraction of a quota, this can be rounded down or rounded to the nearest whole number. Rounding down is equivalent to using the D'Hondt method, while rounding to the nearest whole number is equivalent to the Sainte-Laguë method. However, because of the rounding, this will not necessarily result in the desired number of seats being filled. In that case, the quota may be adjusted up or down until the number of seats after rounding is equal to the desired number. The tables used in the D'Hondt or Sainte-Laguë methods can then be viewed as calculating the highest quota possible to round off to a given number of seats. For example, the quotient which wins the first seat in a D'Hondt calculation is the highest quota possible to have one party's vote, when rounded down, be greater than 1 quota and thus allocate 1 seat. The quotient for the second round is the highest divisor possible to have a total of 2 seats allocated, and so on.
Comparison between the ''D'Hondt'', ''Sainte-Laguë'' and ''Huntington–Hill'' methods
D'Hondt, Sainte-Laguë and Huntington-Hill allow different strategies by parties looking to maximize their seat allocation. D'Hondt and Huntington–Hill can favor the merging of parties, while Sainte-Laguë can favor splitting parties. Examples In these examples, under D'Hondt and Huntington–Hill the Yellows and Greens combined would gain an additional seat if they merged, while under Sainte-Laguë the Yellows would gain if they split into six lists with about 7,833 votes each. The total vote is 100,000. There are 10 seats. The Huntington–Hill method threshold is 10,000, which is 1/10 of the total vote.
