Concept in political party systems
In
political science
, the
effective number of parties
is a
diversity index
introduced by Laakso and Taagepera (1979)
[1]
which provides for an adjusted number of
political parties
in a country's
party system
, weighted by their relative size. The measure is especially useful when comparing
party systems
across countries.
[2]
The size of a party can be measured by either:
- The
effective number of electoral parties
(ENEP)
weights parties by their share of the vote.
- The
effective number of parliamentary parties (ENPP)
weights parties by their share of seats in the legislature.
The number of parties equals the effective number of parties only when all parties have equal strength. In any other case, the effective number of parties is lower than the actual number of parties. The effective number of parties is a frequent
operationalization
for
political fragmentation
.
There are several common alternatives for how to define the effective number of parties.
[3]
John K. Wildgen's index of "hyperfractionalization" accords special weight to small parties.
[4]
Juan Molinar's index gives special weight to the largest party.
[5]
Dunleavy and Boucek provide a useful critique of the Molinar index.
[6]
Measures
[
edit
]
Quadratic
[
edit
]
Laakso
and
Taagepera
(1979) were the first to define the
effective number of parties
using the following formula:
where
n
is the number of parties with at least one vote/seat and
the square of each party's proportion of all votes or seats. This is also the formula for the
inverse Simpson index
, or the true diversity of order 2. This definition is still the most commonly-used in
political science
.
This measure is equivalent to the
Herfindahl?Hirschman index
, used in economics; the
Simpson diversity index
in ecology; and the
inverse participation ratio (IPR)
in physics.
Alternatives
[
edit
]
An alternative formula was proposed by
Grigorii Golosov
in 2010.
[7]
which is equivalent ? if we only consider parties with at least one vote/seat ? to
Here,
n
is the number of parties,
the square of each party's proportion of all votes or seats, and
is the square of the largest party's proportion of all votes or seats.
Values
[
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]
The following table illustrates the difference between the values produced by the two formulas for eight hypothetical vote or seat constellations:
Constellation
|
Largest component, fractional share
|
Other components, fractional shares
|
N, Laakso-Taagepera
|
N, Golosov
|
A
|
0.75
|
0.25
|
1.60
|
1.33
|
B
|
0.75
|
0.1, 15 at 0.01
|
1.74
|
1.42
|
C
|
0.55
|
0.45
|
1.98
|
1.82
|
D
|
0.55
|
3 at 0.1, 15 at 0.01
|
2.99
|
2.24
|
E
|
0.35
|
0.35, 0.3
|
2.99
|
2.90
|
F
|
0.35
|
5 at 0.1, 15 at 0.01
|
5.75
|
4.49
|
G
|
0.15
|
5 at 0.15, 0.1
|
6.90
|
6.89
|
H
|
0.15
|
7 at 0.1, 15 at 0.01
|
10.64
|
11.85
|
Institutional theory
[
edit
]
The effective number of parties can be predicted with the seat product model
[8]
[9]
as
, where
M
is the district magnitude and
S
is the assembly size.
Effective number of parties by country
[
edit
]
For individual countries the values of effective number of number of parliamentary parties (ENPP) for the last available election is shown.
[10]
Some of the highest effective number of parties are in Brazil, Belgium, and Bosnia and Herzegovina. European Parliament has an even higher effective number of parties if national parties are considered, yet a much lower effective number of parties if
political groups of the European Parliament
are considered.
See also
[
edit
]
References
[
edit
]
- ^
Laakso, Markku; Taagepera, Rein (1979).
"
"Effective" Number of Parties: A Measure with Application to West Europe"
.
Comparative Political Studies
.
12
(1): 3?27.
doi
:
10.1177/001041407901200101
.
ISSN
0010-4140
.
S2CID
143250203
.
- ^
Lijphart, Arend (1999): Patterns of Democracy. New Haven/London: Yale UP
- ^
Arend Lijphart
(1 January 1994).
Electoral Systems and Party Systems: A Study of Twenty-seven Democracies, 1945?1990
. Oxford University Press. p.
69
.
ISBN
978-0-19-827347-9
.
- ^
"The Measurement of Hyperfractionalization"
. Cps.sagepub.com. 1971-07-01
. Retrieved
2014-01-05
.
- ^
Molinar, Juan (1 January 1991). "Counting the Number of Parties: An Alternative Index".
The American Political Science Review
.
85
(4): 1383?1391.
doi
:
10.2307/1963951
.
JSTOR
1963951
.
S2CID
154924401
.
- ^
Dunleavy, Patrick; Boucek, Francoise (2003).
"Constructing the Number of Parties"
(PDF)
.
Party Politics
.
9
(3): 291?315.
doi
:
10.1177/1354068803009003002
.
S2CID
33028828
.
- ^
Golosov, Grigorii V. (2010). "The Effective Number of Parties: A New Approach".
Party Politics
.
16
(2): 171?192.
doi
:
10.1177/1354068809339538
.
ISSN
1354-0688
.
S2CID
144503915
.
- ^
Taagepera, Rein (2007). "Predicting Party Sizes". Oxford University Press
- ^
Li, Yuhui; Shugart, Matthew S. (2016).
"The Seat Product Model of the effective number of parties: A case for applied political science"
.
Electoral Studies
.
41
: 23?34.
doi
:
10.1016/j.electstud.2015.10.011
.
- ^
"Election Indices"
(PDF)
.
External links
[
edit
]
- Michael Gallagher providing data on the Laakso-Taagepera effective number of parties for over 900 elections in over 100 countries
- Average effective number of parties (Golosov) for 183 democratic party systems and non-systems, 1792?2009, reported in
Golosov, Grigorii V.
,
"Towards a Classification of the World's Democratic Party Systems, Step 1: Identifying the Units"
, Party Politics, Vol. 19, No. 1, January 2013, pp. 134?138.
- How to compute Golosov’s effective number of parties in Excel