A Semi-proportional voting system is a multi-winner voting system whose proportionality lies between that of majoritarian

Majoritarianism

Majoritarianism is a traditional political philosophy or agenda which asserts that a majority of the population is entitled to a certain degree of primacy in society, and has the right to make decisions that affect the society...

 systems like bloc voting and fully proportional methods like the Sainte-Laguë method

Sainte-Laguë method

The Sainte-Laguë method is one way of allocating seats approximately proportional to the number of votes of a party to a party list used in many voting systems. It is named after the French mathematician André Sainte-Laguë. The Sainte-Laguë method is quite similar to the D'Hondt method, but uses...

 or STV

Single transferable vote

The single transferable vote is a voting system designed to achieve proportional representation through preferential voting. Under STV, an elector's vote is initially allocated to his or her most preferred candidate, and then, after candidates have been either elected or eliminated, any surplus or...

.

The choice to use a semi-proportional voting system may be a deliberate attempt to find a balance between majority rule and proportional representation: semi-proportional systems can allow for fairer representation of those parties that have difficulty gaining individual seats while still keeping the possibility of one party gaining a majority when there is a landslide victory

Landslide victory

In politics, a landslide victory is the victory of a candidate or political party by an overwhelming margin in an election...

. On the other hand, the semi-proportional nature of a legislature may arise unintended, from the dynamics of the system itself.

Because there are many measures of proportionality, and because there is no objective threshold, opinions on what constitutes a semi-proportional method rather than a majoritarian or a fully proportional one, may differ.

Methods where parties can only achieve proportionality by coordinating their voters are usually considered to be semi-proportional. They are not majoritarian, since in the perfect case, the outcome will be proportional, but they are not proportional either, since such a perfect case requires a very high degree of coordination. Such methods include the single non-transferable vote

Single non-transferable vote

The single non-transferable vote or SNTV is an electoral system used in multi-member constituency elections.- Voting :In any election, each voter casts one vote for one candidate in a multi-candidate race for multiple offices. Posts are filled by the candidates with the most votes...

 and limited voting

Limited Voting

Limited voting is a voting system in which electors have fewer votes than there are positions available. The positions are awarded to the candidates who receive the most votes absolutely...

, the latter of which becomes less proportional the more votes each voter has.

The other main group usually considered semi-proportional consists of additional-member systems where the additional members are not apportioned to counter imbalance in party proportionality, or where the manner in which they are so done is not sufficient to balance the disproportionality of the original system. The first subgroup includes parallel voting

Parallel voting

Parallel voting describes a mixed voting system where voters in effect participate in two separate elections for a single chamber using different systems, and where the results in one election have little or no impact on the results of the other...

 and the supplementary member system, and the second, AV+.