Proportional Representation in AP Comparative Government and Politics

Turning vote shares into seats under a party list

In party-list PR, each party puts forward a list of candidates, and seats are handed out based on the party's share of the vote. In a closed-list system, voters pick a party, and the party's ranking decides who sits. If a party earns enough votes for 12 seats, the top 12 names on its list take office. In an open-list system, voters can also back individual candidates, which affects who fills the party's seats. Districts are multimember, so many seats are filled at once, sometimes from one nationwide district.

Many PR systems also set a threshold, which is a minimum vote share a party needs before it gets any seats. Here's a quick example with 100 seats and a 5% threshold. Party A gets 40%, B gets 35%, C gets 15%, D gets 7%, and E gets 3%. Party E falls below the line and gets nothing. The other four split the seats based on their share of the 97% that qualified, which works out to about 41, 36, 16, and 7 seats.

Compare that to single-member district plurality, where a party with 15% of the vote spread evenly across the country could win zero seats because it never comes first in any district. That gap explains most of what PR does to a party system.

Mexico and Russia mix PR with single-member districts

None of the six course countries uses pure PR for its lower house. Mexico and Russia run mixed systems, and the UK and Nigeria use single-member districts only, which makes them the contrast cases.

• Mexico's Chamber of Deputies: 300 members won by plurality in single-member districts plus 200 from PR party lists. In the Senate, 96 seats come from three-seat constituencies and 32 come from PR. Gender quotas on the party lists have helped raise the number of women in Congress.
• Russia's Duma: half the members come from single-member districts and half from PR with a threshold. Russia used all-PR Duma elections for a period before bringing the half-and-half system back, which makes it a common example of a regime adjusting election rules to fit political interests.
• UK House of Commons and Nigeria's House of Representatives: single-member districts with plurality (first-past-the-post) rules, which tend to support two major parties.

The mixed design is a trade-off. The district seats keep local constituency service and clear accountability, and the PR seats let smaller parties and list-based quotas add representation.

Why PR comes up in democratization and coalition questions

PR's main home is Topic 4.2, Objectives of Election Rules, in Unit 4, which asks how rules serve a regime's goals on ballot access, winning elections, and accountability to constituents. Topic 4.1 asks you to describe which course countries use it. PR represents one set of goals (wider representation of parties and groups), while single-member districts represent another (local accountability and geographic representation).

It also reaches back to Topic 1.4, Democratization. The course lists PR, gender or cultural quotas, and changes to thresholds and district lines as rule changes that can make room for ethnic diversity and increase multiparty competition. A rule change can push democratization forward or hold it back, so a raised threshold that keeps opposition parties out works against the goals PR is supposed to serve.

In Unit 2, PR connects to how legislatures and executives work. When many parties win seats and none has a majority, a parliamentary system often needs a coalition government to choose and keep a prime minister and cabinet.

Using PR in a Conceptual Analysis or Comparative Analysis answer

A stem might describe a country switching from single-member districts to PR and ask what most likely happens next, or it might give a quantitative set with vote shares beside seat shares. Multiple-choice questions can also ask about course-country election rules. When seat percentages closely track vote percentages, that points to PR. When one party's seat share is well above its vote share, that points to plurality districts.

On free response, PR fits Question 1 (Conceptual Analysis), where you define the concept and then explain an effect, and Question 3 (Comparative Analysis), which uses examples from two course countries. You could compare Mexico with Russia, two mixed systems that both fill part of the lower house through PR, or compare one of them with the UK or Nigeria, where single-member districts are the whole system. A strong answer gets specific with the numbers (300 and 200 in Mexico, half and half in Russia) and then explains the link: lists and multimember districts let smaller parties and quota-based candidates win seats. On Question 4 (Argument Essay), PR can back a thesis about which election rules best support representation. The opposing view usually rests on accountability, since no single representative answers to one district under pure PR.

One common mistake is giving PR the traits of single-member districts. If a question mentions strong constituency service or guaranteed geographic representation, that's plurality districts. Another mistake is overstating what PR does. The course says it can increase the number of parties and the number of women and minorities elected, so write "can" or "tends to" instead of "guarantees." Also, a mixed system isn't pure PR, so don't describe Mexico or Russia as fully proportional.