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Control Complexity of Schulze Voting

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Control Complexity of Schulze Voting Proceedings of the Twenty-Third International Joint Conference on Artificial Intelligence Control Complexity of Schulze Voting Curtis Menton1 and Preetjot Singh2 1Dept. of Comp. Sci., University of Rochester, Rochester, NY, USA 2EECS, Northwestern University, Evanston, IL, USA Abstract ther characterize the worst-case behavior of Schulze voting under control. We find that it possesses a good but not ex- Schulze voting is a recently introduced voting sys- ceptional number of control resistances, fewer than the best tem enjoying unusual popularity and a high degree known natural systems. Schulze voting currently is known of real-world use, with users including the Wiki- to posses the same control resistances as Copelandα with media foundation, several branches of the Pirate α 2 (0; 1), which was at one point the most resistant system Party, and MTV. It is a Condorcet voting system known that uses the standard linear vote model [Faliszewski that determines the winners of an election using in- et al., 2009b]. While it fails to stand among the most resistant formation about paths in a graph representation of systems, the popularity of the system increases the relevance the election. We resolve the complexity of many of these results, perhaps calling into question the use of the electoral control cases for Schulze voting. We find system in contexts where these control actions are of concern. that it falls short of the best known voting systems in terms of control resistance, demonstrating vul- 1.1 Preliminaries nerabilities of concern to some prospective users of the system. An election is a pair (C; V ) where C is a finite set of candi- dates and V is a finite collection of votes. The most common model is for a vote to be given as a strict linear ordering of 1 Introduction the candidates, though other models are sometimes used as Voting has a long history as a tool for collaborative deci- well. Most notably, approval voting uses binary vectors over sion making. It has classically been used for political and the candidates as votes. organizational elections, but it has become increasingly im- A voting system is a mapping from an election (C; V ) to a 0 portant through its use in artificial intelligence and multia- set C ⊆ C of winners. We do not require a voting system to gent systems. Some applications include recommender sys- select only a single winner: This would require us to build in tems [Ghosh et al., 1999; Pennock et al., 2000], consensus tiebreaking mechanisms while they are better left as separate mechanisms for planning [Ephrati and Rosenschein, 1991] procedures. We do not require a voting system even to always and search engine design [Lifantsev, 2000; Dwork et al., select any winners at all, as there are electoral contexts where 2001]. selecting no candidates is a reasonable outcome. For instance, A persistent worry with elections is that they may be ma- the voting system used to select entrants for the Baseball Hall nipulated, harming their integrity and often calling into ques- of Fame can select no players if none reach a certain threshold tion the authenticity of the results. The seminal work of Gib- of support. bard and Satterthwaite [Gibbard, 1973; Satterthwaite, 1975] (and the later extension of this result [Duggan and Schwartz, 1.2 Schulze Voting 2000]) showed that in fact no reasonable voting system is im- Schulze voting is a Condorcet voting system recently intro- mune to manipulation (i.e., strategic voting). Later, Bartholdi, duced by Marcus Schulze [Schulze, 2011]. It was designed Tovey, and Trick introduced computational complexity as a in part to effectively handle candidate cloning: In many vot- novel defense against this problem [Bartholdi et al., 1989].A ing systems, the inclusion of several similar candidates ends flood of research has subsequently investigated the complex- up diluting their support and lessening the influence of their ity of many voting systems under various manipulative action supporters. It has a somewhat more complex winner proce- problems. dure than other common voting systems, requiring the use of Here we study Schulze voting, a new and sophisticated vot- a graph best-path finding algorithm, but it is still solvable in ing system that has become fairly popular in recent years. We polynomial time, rendering Schulze a tractable voting system. investigate its complexity under control, a class of manipu- Schulze voting has garnered a high level of real-world use, lative action problems where the election chair changes the especially in the free software and free culture communities. structure of the election to affect the result. We build on ear- Users include several national branches of the Pirate Party, lier work by Parkes and Xia [Parkes and Xia, 2012] and fur- the Wikimedia foundation, many free-software projects such 286 as Ubuntu, Debian, Gentoo, and KDE, and even MTV. This 4 a level of real-world usage is unusual for a new, academically b proposed and studied voting system, but it makes Schulze 4 voting more compelling to analyze. 2 2 As a typical Condorcet system, the winners are determined by examining the pairwise contests between candidates. We 6 will thus introduce some useful functions and notation. The c d advantage function is a function on pairs of candidates where adv(a; b) is the number of voters in the given election that Figure 1: Election graph for the example Schulze election. prefer a to b (that is, rank a higher than b in their pref- erences). The net advantage function gives the net differ- ence in advantage for one candidate over another. We de- dates. Note that there are several Condorcet cycles and there fine the net advantage between a and b to be the following: is no clearly dominant candidate: Each of them loses to at netadv(a; b) = adv(a; b) − adv(b; a). least one other candidate, and each candidate has a path to It is easily possible to construct the net advantage function every other candidate. The following are the Schulze best given a collection of votes, but it is interesting to note that path scores. we can easily convert in the other direction as well. Given a b c d a net advantage function where either all the scores are even or all the scores are odd, we can construct an equivalent set a 4 4 4 of votes in time polynomial in the number of candidates and b 2 4 4 in the maximum magnitude of any score in the net advan- c 2 2 6 tage function. This method is due to McGarvey [McGarvey, d 2 2 2 1953]. We will use this method later to more easily construct a elections in the course of reductions, in order to avoid having The only winner of the election will be , as it has a to explicitly specify the votes. stronger path to every other candidate than those candidates have back to a. The winners in a Schulze election are determined as fol- lows. Generate the net advantage scores for the election, and 1.3 Election Manipulative Actions represent this data as a graph, with vertices for candidates and directed edges denoting net advantage scores. Deter- There are several classes of manipulative action problems mine the strongest paths in this graph between every pair of studied in computational social choice. The primary classes candidates by the following metric: The weight of a path is of these problems are manipulation, where a voter or voters the lowest weight edge in the path. This is the “bottleneck” strategically vote to affect the result of an election [Bartholdi metric. We can adapt the Floyd-Warshall dynamic program- et al., 1989], bribery, where an outside briber pays off voters ming algorithm to find the weight of such paths in polynomial to change their votes [Faliszewski et al., 2009a], and control, time [Schulze, 2011]. where an election organizer alters the structure of an election Once we have the best path weights, we build another to change the result [Bartholdi et al., 1992]. graph with the same vertices where there is a directed edge These problems are formalized as decision problems as is from a to b if the best path from a to b is stronger than the standard in computer science and we study their complexity best path from b to a. The winners of the election are the can- in various voting systems. There is some standard terminol- didates with no in-edges in this final graph. That is, they are ogy about the behavior of voting systems under manipulative the candidates that are not beaten by any other candidate in actions. A voting system is immune to a manipulative action relative best-path strength. if the action can never change the result of an election in the Note that a Condorcet winner, in the election graph, will voting system, and the system is susceptible otherwise. If a have positive weight edges to every other candidate, and so voting system is susceptible to some action and the decision they will clearly have the better paths to every other candidate problem is in P, then it is vulnerable to it, and the system and be the only winner of the election. is constructively vulnerable if additionally the corresponding search problem is in P. If the decision problem is NP-hard, the Example Election voting system is resistant to the action. The original versions Consider a Schulze election over candidates fa; b; c; dg with of these problems are the constructive cases, where the goal the following table expressing the net advantage function. is to make a particular candidate win, as opposed to destruc- tive cases, where the goal is to prevent a particular candidate a b c d from winning.
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