Defined as the net change within the electoral party system resulting from individual vote transfers, electoral volatility is measured according to the following formula:
$$Volatility = sum|pi,t-1 - pi,t| / 2$$
in which pi,t is the vote/seat share for the party ith at a given election (t) and pi,t-1 is the vote/seat share of the same party ith at the previous elections (t-1) (Pedersen, 1979).
The index is a number from 0 to 1, 0 means no volatility (proportion of votes/seats remains constant for every party) and 1 means total volatility (every party pass from 0 to any votes or viceversa).
Developed by Jorge Albuja Delgado (albuja@yahoo.com).