Supplementing the effective number of parties

The effective number of parties, N = 1/Sigma p(i)(2) (where p(i) is the fra ctional share of the ith party), usually suffices to describe adequately a constellation of parties of different strengths. Difficulties arise when di sparity in party sizes is such that the largest share (p(1)) surpasses 0.50 (meaning absolute dominance), while N still indicates a multi-party conste llation. In such cases N-infinity = 1/p(1) is proposed as a supplementary i ndicator: a value less than 2 indicates absolute dominance. An 'NP' index p roposed earlier is a combination of N and N-infinity; its values are close to those of N-infinity, but NP sometimes falls below 2 even when many parti es are relevant for coalition formation. Appendix A offers an alternative a pproach based on indices of deviation from a norm, but it proves cumbersome . (C) 1999 Elsevier Science Ltd. All rights reserved.