Paper  doi  bibtex   

@article{ laakso_effective_1979,
  title = {“{Effective}” {Number} of {Parties} {A} {Measure} with {Application} to {West} {Europe}},
  volume = {12},
  issn = {0010-4140, 1552-3829},
  url = {http://cps.sagepub.com/content/12/1/3},
  doi = {10.1177/001041407901200101},
  language = {en},
  number = {1},
  urldate = {2013-08-07TZ},
  journal = {Comparative Political Studies},
  author = {Laakso, Markku and Taagepera, Rein},
  month = {April},
  year = {1979},
  pages = {3--27}
}