3 research outputs found
Algorithmic Techniques for Necessary and Possible Winners
We investigate the practical aspects of computing the necessary and possible
winners in elections over incomplete voter preferences. In the case of the
necessary winners, we show how to implement and accelerate the polynomial-time
algorithm of Xia and Conitzer. In the case of the possible winners, where the
problem is NP-hard, we give a natural reduction to Integer Linear Programming
(ILP) for all positional scoring rules and implement it in a leading commercial
optimization solver. Further, we devise optimization techniques to minimize the
number of ILP executions and, oftentimes, avoid them altogether. We conduct a
thorough experimental study that includes the construction of a rich benchmark
of election data based on real and synthetic data. Our findings suggest that,
the worst-case intractability of the possible winners notwithstanding, the
algorithmic techniques presented here scale well and can be used to compute the
possible winners in realistic scenarios
The Complexity of Determining the Necessary and Possible Top-k Winners in Partial Voting Profiles
When voter preferences are known in an incomplete (partial) manner, winner
determination is commonly treated as the identification of the necessary and
possible winners; these are the candidates who win in all completions or at
least one completion, respectively, of the partial voting profile. In the case
of a positional scoring rule, the winners are the candidates who receive the
maximal total score from the voters. Yet, the outcome of an election might go
beyond the absolute winners to the top- winners, as in the case of committee
selection, primaries of political parties, and ranking in recruiting. We
investigate the computational complexity of determining the necessary and
possible top- winners over partial voting profiles. Our results apply to
general classes of positional scoring rules and focus on the cases where is
given as part of the input and where is fixed
Computing the Extremal Possible Ranks with Incomplete Preferences
Various voting rules are based on ranking the candidates by scores induced by
aggregating voter preferences. A winner (respectively, unique winner) is a
candidate who receives a score not smaller than (respectively, strictly greater
than) the remaining candidates. Examples of such rules include the positional
scoring rules and the Bucklin, Copeland, and Maximin rules. When voter
preferences are known in an incomplete manner as partial orders, a candidate
can be a possible/necessary winner based on the possibilities of completing the
partial votes. Past research has studied in depth the computational problems of
determining the possible and necessary winners and unique winners.
These problems are all special cases of reasoning about the range of possible
positions of a candidate under different tiebreakers. We investigate the
complexity of determining this range, and particularly the extremal positions.
Among our results, we establish that finding each of the minimal and maximal
positions is NP-hard for each of the above rules, including all positional
scoring rules, pure or not. Hence, none of the tractable variants of
necessary/possible winner determination remain tractable for extremal position
determination. Tractability can be retained when reasoning about the top-
positions for a fixed . Yet, exceptional is Maximin where it is tractable to
decide whether the maximal rank is for (necessary winning) but it
becomes intractable for all .Comment: arXiv admin note: substantial text overlap with arXiv:2002.0921