Where to count parties

To evaluate theories about the impact of electoral law on party systems, it is useful to have an indicator of exactly how many parties are competitive in a given election. Several formulae convert the vote shares won by multiple parties into single number-of-parties indices. While these formulae are typically applied to national vote shares, it is often more revealing to examine constituency-level data. For some purposes, the calculated value for the number-of-parties-in-the-nation is a poor substitute for a fuller description of how many parties or candidates exist, constituency by constituency.