Calculates the inverted Grofman index of proportionality based on a population vector and a vector for the representatives

1 | ```
GrofmanInv(Z,R)
``` |

`Z` |
Population vector |

`R` |
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

Didier Ruedin

Taagepera, R., and B. Grofman. Mapping the indices of seats-votes disproportionality and inter-election volatility. Party Politics 9, no. 6 (2003): 659-77.

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