The Pedersen index is a measure of political volatility in party systems. It was described by Mogens Pedersen in a paper published in 1979 entitled The Dynamics of European Party Systems: Changing Patterns of Electoral Volatility. [1]
| Part of the Politics series |
| Elections |
|---|
 |
 Politicsportal |
"The net change within the electoral party system resulting from individual vote transfers" [2]
The Pedersen index is calculated as
where is the percentage of party political party at election and for the consecutive election. To calculate the index, the percentage gains of the winning parties must be determined. The resulting index will be between 0 (no parties gained, and thus no parties lost either) and 100 (all the parties from the last election were reduced to zero votes), because for every gain there is an equal (in terms of percentage of votes) loss. In other words, the index is equal to the net percentage of voters who changed their votes. ("Net percentage," because if the only change is a Party A voter switching to Party B, and a Party B voter switching to Party A, there is no net volatility.) The index can also be constructed by summing the absolute values of all gains and all losses, and dividing this total by two.
The political volatility measured by the Pedersen index differs from the political volatility when parliamentary seats are considered due to the differing seats-to-votes ratios. The Pedersen index can overestimate the political volatility for countries with newly formed parliamentary groups made of previously existing political parties. Other measures [3] differ in their estimates of political volatility.
Assume that in the first election the Blue Party won 65%, the Orange Party won 25%, and the Fuchsia Party won 10%. Furthermore, assume that in the second election the Blue Party won 65%, the Orange Party won 15%, and the Fuchsia Party won 20%.
| Election\Party | Blue | Orange | Fuchsia |
|---|---|---|---|
| 1st | 65% | 25% | 10% |
| 2nd | 65% | 15% | 20% |
| Gain/Loss | 0 | -10 | 10 |
The index would be equal to Blue gains (none) plus Orange's loss (10% since we do not consider sign differences) plus Fuchsia gains (10%). We then multiply it by 1/2 or divide by 2 for a total volatility of 10%.
If all three parties had disappeared in the next election, and been replaced by the Red Party (75%) and the Black Party (25%), the volatility would have been 100%: The first three lose all (100%) + the Red Party gaining 75% and the Black Party 25% since the previous election (when they both received no votes.) 100+100 = 200 -> divide by 2 = 100
The Pedersen indices for individual countries [4] are listed below, only the last available index is shown. The Pedersen index tends to decrease for some countries with increasing number of consecutive elections. [4]
| Country | Year | Pedersen index |
|---|---|---|
 Argentina | 2011 | 25.1 |
 Austria | 2010 | 7.3 |
 Australia | 2010 | 7.3 |
 Bolivia | 2009 | 35.9 |
 Brazil | 2010 | 18.2 |
 Bulgaria | 2009 | 39.9 |
 Chile | 2009 | 13.9 |
 Colombia | 2010 | 15.9 |
 Costa Rica | 2010 | 29.0 |
 Czech Republic | 2010 | 27.7 |
 Dominican Republic | 2010 | 32.3 |
 Ecuador | 2009 | 33.0 |
 Estonia | 2011 | 32.5 |
 El Salvador | 2012 | 15.5 |
 Germany | 2009 | 8.3 |
 Honduras | 2009 | 7.8 |
 Hungary | 2010 | 25.1 |
 Israel | 2009 | 20.9 |
 India | 2009 | 25.1 |
 Italy | 2008 | 15.2 |
 Japan | 2009 | 14.2 |
 Latvia | 2011 | 36.4 |
 Lithuania | 2012 | 39.7 |
 Macedonia | 2011 | 32.0 |
 Malaysia | 2008 | 13.7 |
 Mexico | 2009 | 21.0 |
 Mongolia | 2008 | 24.3 |
 Netherlands | 2010 | 13.4 |
 Paraguay | 2008 | 25.6 |
 Romania | 2008 | 36.9 |
 Sweden | 2009 | 8.4 |
 Singapore | 2011 | 10.8 |
 South Korea | 2012 | 29.3 |
 Taiwan | 2012 | 16.9 |
 Thailand | 2010 | 27.2 |
 Trinidad and Tobago | 2011 | 25.1 |
 United Kingdom | 2010 | 7.6 |
 United States | 2010 | 3.4 |
 Uruguay | 2009 | 14.6 |
 Venezuela | 2010 | 34.5 |
Proportional representation (PR) refers to any type of electoral system under which subgroups of an electorate are reflected proportionately in the elected body. The concept applies mainly to political divisions among voters. The essence of such systems is that all votes cast – or almost all votes cast – contribute to the result and are effectively used to help elect someone. Under other election systems, a bare plurality or a scant majority are all that are used to elect candidates. PR systems provide balanced representation to different factions, reflecting how votes are cast.
The electoral threshold, or election threshold, is the minimum share of votes that a candidate or political party requires before they become entitled to representation or additional seats in a legislature.
The D'Hondt method, also called the Jefferson method or the greatest divisors method, is an apportionment method for allocating seats in parliaments among federal states, or in proportional representation among political parties. It belongs to the class of highest-averages methods. Compared to ideal proportional representation, the D'Hondt method reduces somewhat the political fragmentation for smaller electoral district sizes, where it favors larger political parties over small parties.
The Webster method, also called the Sainte-Laguë method, is a highest averages apportionment method for allocating seats in a parliament among federal states, or among parties in a party-list proportional representation system. The Sainte-Laguë method shows a more equal seats-to-votes ratio for different sized parties among apportionment methods.
The Unionist Party was the main centre-right political party in Scotland between 1912 and 1965.
There are three types of elections in Denmark: elections to the national parliament, local elections, and elections to the European Parliament. Referendums may also be called to consult the Danish citizenry directly on an issue of national concern.
In the study of apportionment, the Harequota is the number of voters represented by each legislator under an idealized system of proportional representation, where every legislator represents an equal number of voters and where every vote is used to elect someone. The Hare quota is the total number of votes divided by the number of seats to be filled. The Hare quota was used in the original proposal for a single transferable vote system, and is still occasionally used, although it has since been largely supplanted by the Droop quota.
Starting with the 2000 United States presidential election, the terms "red state" and "blue state" have referred to US states whose voters vote predominantly for one party—the Republican Party in red states and the Democratic Party in blue states—in presidential and other statewide elections. By contrast, states where the vote fluctuates between the Democratic and Republican candidates are known as "swing states" or "purple states". Examining patterns within states reveals that the reversal of the two parties' geographic bases has happened at the state level, but it is more complicated locally, with urban-rural divides associated with many of the largest changes.
An electoral swing analysis shows the extent of change in voter support, typically from one election to another, expressed as a positive or negative percentage. A multi-party swing is an indicator of a change in the electorate's preference between candidates or parties, often between major parties in a two-party system. A swing can be calculated for the electorate as a whole, for a given electoral district or for a particular demographic.
The Gallagher index measures an electoral system's relative disproportionality between votes received and seats in a legislature. As such, it measures the difference between the percentage of votes each party gets and the percentage of seats each party gets in the resulting legislature, and it also measures this disproportionality from all parties collectively in any one given election. That collective disproportionality from the election is given a precise score, which can then be used in comparing various levels of proportionality among various elections from various electoral systems. The Gallagher index is a statistical analysis methodology utilised within political science, notably the branch of psephology.
In electoral systems, a wasted vote is any vote cast that is not "used" to elect a winner, and so is not represented in the outcome. However, the term is vague and ill-defined, having been used to refer to a wide variety of unrelated concepts and metrics. The precise definition of a wasted vote can have a major impact on the conclusions of an analysis. For example, under the narrowest possible definition of a wasted vote, the single transferable vote (STV) can be considered to waste zero votes. However, if the wasted vote definition is expanded even slightly, it is possible for up to 100% of STV votes to be classified as wasted because STV fails the unanimity criterion; that is, it is possible to elect a legislature that every single voter agrees is worse than some alternative.
Apportionment is the process by which seats in a legislative body are distributed among administrative divisions, such as states or parties, entitled to representation. This page presents the general principles and issues related to apportionment. The page apportionment by country describes the specific practices used around the world. The page Mathematics of apportionment describes mathematical formulations and properties of apportionment rules.
The 1940 United States presidential election in California took place on November 5, 1940, as part of the 1940 United States presidential election. State voters chose 22 representatives, or electors, to the Electoral College, who voted for president and vice president.
Swing, in the politics of the United Kingdom, is a number used as an indication of the scale of voter change between two political parties. It originated as a mathematical calculation for comparing the results of two Parliamentary constituencies. The UK uses a first-past-the-post voting system. The swing is the percentage of voter support minus the comparative percentage of voter support corresponding to the same electorate or demographic.
The 1920 United States presidential election in New Mexico took place on November 2, 1920. All contemporary forty-eight States were part of the 1920 United States presidential election. Voters chose three electors to represent them in the Electoral College, which voted for President and Vice President.
The Sainte-Laguë index (SLI) measures an election’s disproportionality, the adherence to the one person, one vote principle of equal representation. This index assumes if the fraction of voters matches the fraction of seats, then perfect proportionality is achieved.
A major branch of social choice theory is devoted to the comparison of electoral systems, otherwise known as social choice functions. Viewed from the perspective of political science, electoral systems are rules for conducting elections and determining winners from the ballots cast. From the perspective of economics, mathematics, and philosophy, a social choice function is a mathematical function that determines how a society should make choices, given a collection of individual preferences.
The 1892 United States presidential election in Georgia took place on November 8, 1892, as part of the wider United States presidential election. Voters chose 13 representatives, or electors, to the Electoral College, who voted for president and vice president.
Apportionment in the Hellenic Parliament refers to those provisions of the Greek electoral law relating to the distribution of Greece's 300 parliamentary seats to the parliamentary constituencies, as well as to the method of seat allocation in Greek legislative elections for the various political parties. The electoral law was codified for the first time through a 2012 Presidential Decree. Articles 1, 2, and 3 deal with how the parliamentary seats are allocated to the various constituencies, while articles 99 and 100 legislate the method of parliamentary apportionment for political parties in an election. In both cases, Greece uses the largest remainder method.
The seats-to-votes ratio, also known as the advantage ratio, is a measure of equal representation of voters. The equation for seats-to-votes ratio for a political party i is:
