DISPROPORTIONALITY INDEXES AND ROBUSTNESS OF PROPORTIONAL ALLOCATION METHODS

Proportional electoral formulae minimize the difference between the vo te and seat share of each party but some methods are considered 'more proportional' than others. The reason can usually be attributed to the vagueness on what should be considered as a correct measure of propor tionality. In fact, electoral formulae and measures of disproportional ity are strictly related: each formula yields a seat assignment which minimizes a precise disproportionality index. Thus, the common procedu re of testing proportionality of all methods using one of such indexes can be misleading. We propose to evaluate the proportionality of elec toral formulae in terms of their robustness. The results show that whi le no method dominates the others in terms of proportionality, Sainte- Lague and LR-Hare tend to be 'robust' respect to the set of indexes he re chosen. (C) 1998 Elsevier Science Ltd. All rights reserved.