Calculates the inverted Grofman index of proportionality based on a population vector and a vector for the representatives
Vector for the representatives
The order of the groups in he population vector and the vector for the representatives needs to be the same. If there are groups A, B, C in the population, both vectors need to cover their proportion in the same order.
The inverted Grofman index is 1 minus the sum the absolute differences between the proportion of the population and representatives in each group. A division by the effective number of groups is applied.
A single score given the population and representatives
Taagepera, R., and B. Grofman. Mapping the indices of seats-votes disproportionality and inter-election volatility. Party Politics 9, no. 6 (2003): 659-77.