The Pedersen index is a measure of electoral volatility in
party
A party is a gathering of people who have been invited by a host for the purposes of socializing, conversation, recreation, or as part of a festival or other commemoration or celebration of a special occasion. A party will often feature ...
systems. It was described by
Mogens Pedersen in a paper published in 1979 entitled ''The Dynamics of European Party Systems: Changing Patterns of
Electoral
An election is a formal group decision-making process by which a population chooses an individual or multiple individuals to hold public office.
Elections have been the usual mechanism by which modern representative democracy has operated ...
Volatility''.
What the index means
"The net change within the electoral party system
A party system is a concept in comparative political science concerning the system of government by political parties in a democratic country. The idea is that political parties have basic similarities: they control the government, have a stab ...
resulting from individual vote
Voting is a method by which a group, such as a meeting or an Constituency, electorate, can engage for the purpose of making a collective decision making, decision or expressing an opinion usually following discussions, debates or election camp ...
transfers"
[W. Ascher and S. Tarrow, 'The Stability of Communist Electorates: Evidence from a Longitudinal Analysis of French and Italian Aggregate Data', American Journal of Political Science, 19/3 (1975), 48o-i.]
Construction of the Index
To calculate the
index
Index (or its plural form indices) may refer to:
Arts, entertainment, and media Fictional entities
* Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index''
* The Index, an item on a Halo megastru ...
, 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
vote
Voting is a method by which a group, such as a meeting or an Constituency, electorate, can engage for the purpose of making a collective decision making, decision or expressing an opinion usually following discussions, debates or election camp ...
s), 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
Net or net may refer to:
Mathematics and physics
* Net (mathematics), a filter-like topological generalization of a sequence
* Net, a linear system of divisors of dimension 2
* Net (polyhedron), an arrangement of polygons that can be folded u ...
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.
Example
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%.
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
References
{{DEFAULTSORT:Pedersen Index
Political science theories