Electoral volatility measures the degree of vote switching between political parties in two consecutive elections. Political scientists use this as an indicator of party system (in)stability. Pedersen (1979) states that volatility should increase when the number of parties changes and/or relevant parties experience vote transfer between elections. However, his proposed functional form of measuring volatility does not always respond to these changes. To address these limitations, we introduce a class of additively separable electoral volatility measures which are responsive to changes in both the number of parties and their vote shares. We present a set of axioms that are both necessary and sufficient to characterize the proposed class of indices, making the structure of the indices more transparent. The paper also introduces two quasi orders which can rank party systems in terms of all electoral volatility indices satisfying certain intuitively reasonable axioms. Finally, applications of the proposed class of indices and the quasi orders are provided using data from Indian state elections.

选举波动性衡量连续两次选举中政党之间选票转换的程度。政治学家将此作为政党制度稳定性的指标。Pedersen（1979）指出，当政党数量发生变化和/或相关政党在选举之间经历选票转移时，波动性就会增加。然而，他提出的衡量波动性的函数形式并不总是对这些变化做出反应。为了解决这些限制，我们引入了一类可加性分离的选举波动性指标，这些指标能够响应政党数量及其投票份额的变化。我们提出了一组公理，这些公理对于描述所提议的指数类别来说既必要又充分，从而使指数的结构更加透明。本文还引入了两个准顺序，可以根据满足某些直观合理公理的所有选举波动性指数对政党系统进行排名。最后，使用印度州选举的数据提供了所提议的指数类别和准订单的应用。