Pedersen index
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The Pedersen index is a measure of electoral volatility in party systems. It was described by Mogens Pedersen in a paper published in 1979 entitled The Dynamics of European Party Systems: Changing Patterns of Electoral Volatility.
[edit] Construction of the Index
To calculate the index, you must sum the gains of winning parties (in terms of % of votes cast) together. The resultant index will be between 0 (no parties gained, and thus no parties lost either) and 100 (all the parties from the last election were reduced to zero votes), because for every gain there is an equal (in terms of percentage of votes) loss. In other words, the index is equal to the net percentage of voters who changed their votes. ("Net percentage," because if the only change is a Party A voter switching to Party B, and a Party B voter switching to Party A, there is no net volatility.) The index can also be constructed by summing the absolute values of all gains and all losses, and dividing this total by two.
[edit] Example
Assume that in the first election the Blue Party won 65%, the Orange Party won 25%, and the Fuchsia Party won 10%. Furthermore, assume that in the second election the Blue Party won 65%, the Orange Party won 15%, and the Fuchsia Party won 20%.
| Blue Party | Orange Party | Fuchsia Party | |
| 1st election | 65% | 25% | 10% |
| 2nd election | 65% | 15% | 20% |
| gain | 0 | 0 | 10 |
The index would be equal to Blue gains (none) plus Orange gains (none; losses not counted) plus Fuchsia gains (10%) for a total volatility of 10%.
If all three parties had disappeared in the next election, and been replaced by the Red Party (75%) and the Black Party (25%), the volatility would have been 100%: the Red Party gaining 75% and the Black Party 25% since the previous election (when they both received no votes.)
