Robustness among multiwinner voting rules

We investigate how robust the results of committee elections are with respect to small changes in the input preference orders, depending on the voting rules used. We find that for typical rules the effect of making a single swap of adjacent candidates in a single preference order is either that (1) at most one committee member might be replaced, or (2) it is possible that the whole committee will be replaced. We also show that the problem of computing the smallest number of swaps that lead to changing the election outcome is typically NP-hard, but there are natural FPT algorithms. Finally, for a number of rules we assess experimentally the average number of random swaps necessary to change the election result.

Authors

• Robert Bredereck TU Clausthal, Institut für Informatik
• Piotr Faliszewski AGH University of Krakow
• Andrzej Kaczmarczyk Czech Technical University of Prague
• Rolf Niedermeier
• Piotr Skowron
• Nimrod Talmon Ben-Gurion University of the Negev

Keywords

• Computational social choice
• Computational complexity
• Parameterized complexity
• Winner determination
• STV
• Copeland
• Committee scoring rules
• Robustness radius
• Robustness level