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The main function of deviation from proportionality is to denote the proportion of malapointment in a democratic process. The idea is basically a mathematical relationship between the percentage of votes obtained by a political party and the percentage of parliamentary seats allocated to it.
To calculate just subtract a parties’ vote share from their seat share, add up all of the absolute values (that is, ignoring any negative signs), and finally divide by two.
- ^ There are three main ways of measuring deviations from proportionality: