Properties of multiwinner voting rules

A committee selection rule (or, multiwinner voting rule) is a mapping that takes a collection of strict preference rankings and a positive integer k as input, and outputs one or more subsets of candidates of size k. In this paper we consider committee selection rules that can be viewed as generalizations of single-winner scoring rules, including SNTV, Bloc, k-Borda, STV, as well as several variants of the Chamberlin–Courant rule and the Monroe rule and their approximations. We identify two natural broad classes of committee selection rules, and show that many of the existing rules belong to one or both of these classes. We then formulate a number of desirable properties of committee selection rules, and evaluate the rules we consider with respect to these properties.


Introduction
There are many situations where societies need to select a small set of entities from a larger group.For example, in indirect democracies people choose representatives to govern on their behalf, companies select groups of products to promote to their customers [37,38], web search engines decide which pages to display for a given query [21], and applicants for a job (e.g., a tenure-track position at a university) are short-listed prior to conducting interviews.For all these tasks we need formal rules to perform the selection, and the desirable properties of such rules may depend on the task at hand.We view these selection rules as multiwinner voting rules which, given individual preferences, output groups of winners (which we call committees).
Multi-winner elections are even more ubiquitous than single-winner ones, but much less studied.They were implicitly considered under the umbrella of choice functions [1,24] but in this model the size of the elected committee could not be controlled and it was Debord [19] and Felsenthal and Maoz [23] who introduced several k-choice functions that elect committees of size exactly k, and investigated their properties.However even within this narrower framework several quite distinct models happily coexist, which makes a simultaneous study of them difficult.One obvious classification is based on the type of the input.There are preference-based rules (for which inputs are sequences of linear orders; see, e.g., the work of Brams and Fishburn [10]), approval-based rules (for which inputs are sequences of dichotomies; see, e.g., the overview of Kilgour [32]), tournament-based rules (for which inputs are either tournaments or weighted tournaments; see the book of Laslier for a general overview of tournament-based rules [35]).With a few exceptions, majority of papers that study (properties of) multiwinner rules are devoted to approval-based rules [2,11,14,32,33] and to rules based on various forms of the Condrocet principle [22,25,27,41].On the other hand, our paper focuses on rules that, in some broader sense, can be seen as extensions of positional scoring rules.Our goal is to review some natural preference-based multiwinner rules, to present a uniform framework for their study, and to propose a set of natural properties (axioms) against which these rules can be judged.In effect, we focus on the model where voters have ordinal preferences.
We have picked ten voting rules as examples of different ideas pertaining to (scoringbased) multiwinner elections: STV, SNTV, k-Borda, Bloc, three variants of Chamberlin-Courant's rule [7,15,37], and three variants of Monroe's rule [7,39,46].STV and SNTV are well-known rules that are used for parliamentary elections in some countries; 1 Bloc is a rule that asks voters to specify their favorite committee of k candidates and selects those k candidates that were nominated more often than others; k-Borda picks k alternatives with the highest Borda scores and is representative of rules used for picking k finalists in a competition (indeed, Formula 1 racing and Eurovision song contest use scoring rules very similar to Borda).Chamberlin-Courant's rule and Monroe's rule are examples of rules that, like STV, focus on proportional representation, but are based on explicitly assigning a committee member (a representative) to each voter.We also consider two rules based on approximation algorithms, for the Chamberlin-Courant's rule [37] and for the Monroe's rule [46].We consider them as voting rules in their own right.All these rules can be seen as being loosely based, in some way, on single-winner scoring protocols.
We are interested in judging the selected multiwinner rules with respect to their applicability in the following settings: Parliamentary Elections.Voting rules for such elections should respect the "one person, one vote" principle.This is reflected in the requirement that each elected member should represent, roughly, the same number of voters.Some such rules are based on electoral districts, i.e., separate (possibly multiwinner) elections are held in different parts of the country, while others treat the whole country as a single constituency, and focus on proportional representation of different population groups.
Shortlisting.Consider a situation where a position is filled at a university.Each faculty member ranks applicants in order to create a short-list of those to be invited for an interview.One of the important requirements in this case is that if some candidate is shortlisted when k applicants are selected, then this candidate should also be shortlisted if the list were extended to k + 1 applicants.
Movie selection.Based on rankings provided by different customer groups, an airline has to decide which (few) movies to offer on their long-distance flights.It is important that each passenger finds something satisfying.This task is similar to parliamentary elections, but without the need to worry that each movie would be watched by the same number of people.It is, however, quite different from shortlisting: If there are two similar candidates, then for shortlisting we should, typically, take either both or neither, whereas in the context of movie selection it makes sense to pick at most one of them.Skowron et al. [45] expand this view of multiwinner elections and provide a number of other examples and applications.
We study properties of voting rules that are important in the above-listed settings.We introduce committee monotonicity, solid coalitions property, consensus committee property, and unanimity, and adapt the standard notions of monotonicity, homogeneity, and consistency to the multiwinner framework.We discuss related literature and compare our approaches in Section 9.
Our paper is a preliminary attempt to give a formal framework for the study of preference-based multiwinner rules.Thus we use the word axiom quite freely, without meaning that it should be a normative requirement.Our work focuses on multiwinner rules that are based on scoring protocols.Such rules are, in some deep sense, very different from those that are based on the Condorcet criterion (indeed, in the single-winner case, no scoring protocol is Condorcet consistent; in effect, a multiwinner rule that degenerates to the case of a scoring rule for single-member committees cannot be Condorcet consistent in any natural way).It would be very interesting to apply our framework to Condorcet-based rules.We leave this as future work.
The paper is organized as follows.In Section 2 we introduce the basic terminology used in this paper; in Section 3 we define the rules that we study and put forward two ways of classifying them.In Section 4, we define several properties of multiwinner rules and in Sections 5-8 we study particular groups of these properties in detail.We present related literature in Section 9 and conclude in Section 10.

Preliminaries
An election is a pair E = (C, V ), where C = {c 1 , . . ., c m } is a set of candidates and V = (v 1 , . . ., v n ) is a sequence of voters.Each voter is described by a preference order, which is a ranking of the candidates from the most desirable one to the least desirable one.We denote the position of a candidate c ∈ C in the preference order of a voter v ∈ V by pos v (c).If V 1 and V 2 are two sequences of voters over the same candidate set C, then V 1 +V 2 denotes the concatenation of V 1 and V 2 .If V is a sequence of voters and t is an integer, then tV denotes the concatenation of t copies of V .For , and for E = (C, V ) and a positive integer t, we write tE to denote (C, tV ).For an integer n, we denote {1, . . ., n} by [n].
A multiwinner voting rule R is a function that given an election E = (C, V ) and a positive integer k, k ≤ C , returns a set R(E, k) of k-element subsets of C, which we call committees.That is, a rule returns a set of committees that are tied-for-winning.In practice, one would need to combine such a rule with a tie-breaking mechanism but, for simplicity, we mostly disregard this issue here.Brams and Fishburn [10] introduced choose-k rules but their definition stipulates that such a rule selects at least k alternatives.Two early papers that focus on rules selecting committees of size exactly k are those of Debord [19] and of Felsenthal and Maoz [23].We use the same approach and when we need to emphasize the size of the committee, we use the term k-committee selection rules.
Requiring multiwinner rules to pick committees of exactly a given size is natural if, for example, the goal is to elect a parliament whose size is fixed by the constitution.However, as a consequence, we are sometimes forced to elect Pareto-dominated candidates (e.g., if all voters unanimously rank the candidates in the same order and k > 1).Alternatively, we could require R(E, k) to return committees of up-to-k members.The latter approach is also studied in the literature (either explicitly or implicitly), but we adopt the former one due to its simplicity and applicability in our settings of interest.

Multiwinner Voting Rules
We now provide definitions of the multiwinner rules that we study and discuss two general ways of defining them.

Common Multiwinner Rules
Many multiwinner rules rely on ideas from single-winner rules, so let us review these first.Many single-winner rules calculate the scores of alternatives to decide which one is best.Here are some most popular ways of computing candidate scores.
Given these definitions, we are ready to describe the multiwinner rules that we focus on in this paper.Let E = (C, V ) be an election and let k ∈ [ C ] be the size of the committee that we seek.We assume the parallel-universes tie-breaking [16], i.e., our rules return all the committees that could result from some breaking of the ties occurring during the computation of the rule.
Single Transferable Vote (STV).STV is a multistage elimination rule that works as follows.If there is a candidate c whose Plurality score is at least q = V k+1 + 1 (the so-called Droop quota), we do the following: (a) include c in the winning committee, (b) delete q votes where c is ranked first, and (c) remove c from all the remaining votes.If each candidate's Plurality score is less than q, a candidate with the lowest Plurality score is deleted from all votes.(There are also many other variants of STV; we point the reader to the work of Tideman and Richardson [48] for details.) Single Nontransferable Vote (SNTV).Under SNTV, we return the k candidates with the highest Plurality scores (thus one can think of SNTV as simply k-Plurality).
Bloc.Under Bloc, we return the k candidates with the highest k-approval scores.
k-Borda.Under k-Borda, we return the k candidates with the highest Borda scores.Debord [18] provided an axiomatic characterization of this rule.
Chamberlin-Courant's and Monroe's Rules.These rules explicitly aim at proportional representation.The main idea is to provide an optimal assignment of committee members to voters by using a satisfaction function to measure the quality of the assignment.
A satisfaction function is a nonincreasing mapping α : N → N. Intuitively, α(i) is a voter's satisfaction from being represented by a candidate that this voter ranks in position i.We focus on the Borda satisfaction function, which for m candidates is defined as α m B (i) = m − i.Let k be the target size of the committee.A function Φ : V → C is called an assignment function and it is called a k-assignment function if Φ(V ) ≤ k.Intuitively, Φ(V ) is the elected committee where voter v is represented by candidate Φ(v).There are several ways to compute the societal satisfaction from the assignment; we focus on the following two: where α is the given satisfaction function.The former one, ℓ 1 (Φ), is a utilitarian measure, which sums the satisfactions of all the voters, and the latter one, ℓ min (Φ), is an egalitarian measure, which consider the satisfaction of the least satisfied voter.
Let α be a satisfaction function and let ℓ be ℓ 1 or ℓ min .Chamberlin-Courant's rule with parameters ℓ and α (ℓ-α-CC) finds an assignment function Φ that maximizes ℓ(Φ) and declares the candidates in Φ(V ) to be the winning committee.If Φ(V ) < k, the rule fills in the missing committee members in an arbitrary way and outputs all resulting committees.ℓ-α-Monroe's rule is defined in the same way, except that we optimize over assignment functions that additionally satisfy the so-called Monroe criterion, which requires that ⌊ n k ⌋ ≤ Φ −1 (c) ≤ ⌈ n k ⌉ for each elected candidate c.To simplify notation, we omit α m B when referring to Monroe/CC rule with the Borda satisfaction function.
For Chamberlin-Courant's rule, for each set of candidates C ′ ⊆ C we define the assignment function Φ CC (C ′ ) so that for each voter v, If W is a winning committee under Chamberlin-Courant's rule, then Φ CC (W ) is an optimal assignment function.
Approximate Variants of ℓ 1 -Monroe and ℓ 1 -CC.Hardness results for ℓ 1 -CC and ℓ 1 -Monroe inspired research on designing efficient approximation algorithms for these rules [37,46].Here, in the spirit of Caragiannis et al. [13], we consider these algorithms as full-fledged multiwinner rules.
We refer to the rules based on approximation algorithms for ℓ 1 -CC and ℓ 1 -Monroe as Greedy-CC and Greedy-Monroe, respectively.Greedy-CC was proposed by Lu and Boutilier [37] and Greedy-Monroe by Skowron et al. [46].Both rules use the Borda satisfaction function, aggregated in the utilitarian way, using ℓ 1 .They proceed in k iterations, in which they build sets and declare W k to be the winning committee.In the i-th iteration, i In particular, c 1 is an alternative with the highest Borda score.
Greedy-Monroe, in addition to the sets W 0 , . . ., W k , also maintains sets of voters such that, after the i-th iteration, V i is the set of voters for which the rule has already assigned candidates.In the i-th iteration, the rule picks a number n i ∈ {⌈ n k ⌉, ⌊ n k ⌋} (see below for the choice criterion) and then picks a candidate intuitively, Greedy-Monroe assigns c i to the voters in V ′ ).Regarding the choice of n i , if n is of the form kn ′ + n ′′ , where 0 ≤ n ′′ < k, then Greedy-Monroe picks ⌈ n k ⌉ for the first n ′′ iterations and picks ⌊ n k ⌋ for the remaining ones.In particular, c 1 is the Borda winner of a group V 1 of n 1 voters which maximizes the Borda score across all subsets of voters of size n 1 .
Greedy-CC and Greedy-Monroe output committees that approximate those output by ℓ 1 -CC and ℓ 1 -Monroe.In particular, Greedy-CC finds a committee W such that the satisfaction of the voters is at least 1 − 1 e of the satisfaction achieved under ℓ 1 -CC [37], and Greedy-Monroe finds a committee that achieves at least a 1 − k 2m−1 − H k k fraction of the satisfaction given by ℓ 1 -Monroe, where k [46] (for many practical setting, this value is quite close to 1).These rules are efficiently computable in the sense that we can output some winning committee in polynomial time; however, their computational complexity under parallel-universes tie-breaking is not known.
Hardness of Winner Determination.For ℓ 1 -Monroe and ℓ 1 -CC, their hardness of winner determination is known [7,37,40,46].Other rules that we study, on the surface, are polynomial-time computable.However, since in this paper we very broadly apply the parallel-universes tie-breaking, this polynomial-time complexity is not necessarily obvious.For SNTV, Bloc, and k-Borda, even with this tie-breaking, winner determination is computationally easy.On the other hand, for STV the problem is NP-hard [16].For Greedy-CC and Greedy-Monroe the answer is currently unclear, but for both of these rules there are simple tie-breaking mechanisms under which they are polynomial-time computable and which do not break any of the axiomatic properties that we study.

Two Types of Multiwinner Rules
Perhaps surprisingly, it turns out that many of the rules introduced so far have very similar internal structure.Below we present two natural ways of identifying these similarities.
Best-k Rules.SNTV and k-Borda are natural extensions of Plurality and Borda to the multiwinner setting: We sort the candidates in the order of decreasing scores (with further parallel-universes tie-breaking if needed) and pick the top k ones.
We remind the reader that a social preference function F is a function that, given an election E = (C, V ), returns a set of tied linear orders over C and a social welfare function returns just one but maybe non-strict linear order.Breaking ties in the latter using the parallel-universes tie-breaking we can also convert a social welfare function into a social preference function (in effect, we will often treat social welfare functions as special cases of social preference functions).Definition 1.We say that a given multiwinner rule R is a best-k rule if there is a social preference function SNTV and k-Borda are best-k rules.Indeed, a class of best-k rules can be defined by providing a score vector s = (s 1 , . . ., s m ), where s 1 ≥ s 2 ≥ . . .≥ s m .This defines a social welfare function which ranks all alternatives in accord with their s-scores, which gives us a best-k rule.As we will see later, perhaps unexpectedly, Bloc is not a best-k rule.
We can also define a best-k rule based on the social preference function known as the Kemeny ranking [31], and, somewhat surprisingly, we will later note that Greedy-CC is also a best-k rule.Thus, best-k rules are a more diverse group than one might at first expect.
Committee Scoring Rules.Both k-Borda and ℓ 1 -CC can be viewed as generalizations of the Borda rule to the multiwinner case.Here we introduce a class of committee scoring rules, which generalize single-winner scoring rules capturing k-Borda, ℓ 1 -CC, and many other rules.We believe that identifying committee scoring rules is an important conceptual contribution of this paper.
Consider an election E = (C, V ) where we want to pick a committee of size k out of m = C candidates.A k-winner committee scoring rule is defined via a committee scoring function f , f : [m] k → N, as follows.Given a committee S and a voter v, we define pos v (S) to be the vector (i 1 , . . ., i k ) resulting from sorting the set {pos v (c) | c ∈ S} in the nondecreasing order.The winning committees are the ones that maximize the sum: which we call the score of the committee S under f .
Just as for single-winner scoring rules, we require a certain form of monotonicity with respect to the values of f .Specifically, if I = (i 1 , . . ., i k ) and J = (j 1 , . . ., j k ) are two increasing sequences of numbers from [m], we say that I J if and only if (i 1 ≤ j 1 ) ∧ . . .∧ (i k ≤ j k ), and we require that I J implies f (I) ≥ f (J).
Example 1.Let m be the number of candidates and let k be the target size of the committee.Define SNTV, Bloc, k-Borda, and ℓ 1 -CC are committee scoring rules defined by: We note that in the case of f SNTV and f k-Borda , the function f has the form: for some non-increasing function γ not depending on k.This is immediate for f k-Borda and for f SNTV it follows by the nature of α 1 .On the other hand, f CC is clearly not of the given form, and neither is f Bloc because function α k depends on k.We refer to committee scoring functions of the form (1) as separable.
Let us focus on a separable committee scoring rule with a scoring function of the form (1), and let us denote γ(i) = s i , i = 1, . . ., m.We form a scoring vector s = (s 1 , . . .s m ); since γ is non-increasing, we have s 1 ≥ s 2 ≥ • • • ≥ s m and, so, s is indeed a valid scoring vector.Now, given some committee S = {a 1 , . . ., a k }, it is easy to verify that the total score of S at some profile Hence, the voting rule is the best-k rule induced by the scoring social welfare function defined by the score vector s.In effect, we have the following theorem.
Theorem 1.Every separable k-committee scoring rule is a k-best rule for some scoring social welfare function.
According to our definition, Bloc is not a separable committee scoring rule and, in some ways, it is quite distinct from separable rules (e.g., it is not a best-k rule; we give another example that Bloc stands out at the end of Section 4).On the other hand, at the formal level Bloc does look very similar to separable committee scoring rules and it shares a number of features with them.To capture this fact, we introduce the notion of a weakly separable committee scoring rule.Definition 2. We say that a committee scoring rule R, defined through scoring function f , is weakly separable if there exists a family The main difference between separable committee scoring rules and weakly separable ones is that weakly separable ones allow function f to use a score vector that depends on k (as in the case of Bloc).
Weakly separable committee scoring rules form an interesting class of multiwinner rules.For example, It is immediate to see that they are polynomial-time computable (provided that they are defined through a family of polynomial-time computable scoring function) and in Section 7 we show that they have an interesting monotonicity property.Proposition 2. If R is a weakly separable committee scoring rule defined through a polynomial-time computable family of functions (in the sense of Definition 2), then R is polynomial-time computable.
On the other extreme, we have committee scoring rules defined through functions f (i 1 , . . ., i k ) whose values depend solely on i 1 (like the fourth rule in Example 1).Such rules seem to focus on voter representation.As in ℓ 1 -CC, each voter is assigned to her most preferred candidate in the selected committee, and only contributes towards the score of this candidate.We call such committee scoring rules representation-focused.
It seems that representation-focused rules, in essence variants of the Chamberlin-Courant rule, are NP-hard to compute [7,37,40], but there is at least one exception: We note that SNTV is both separable and representation-focused and, in effect, is polynomialtime computable.One could say that this is because of its separability.While this indeed is a convincing explanation, there also are non-separable committee scoring rules that are polynomial-time computable, with Bloc being perhaps the simplest example (though, Bloc is weakly separable).

Axioms
We now put forward some properties (axioms) that multiwinner rules may or may not satisfy.We use the standard axioms for single-winner rules as our starting point, and augment them with ideas from the literature that are specific to the multiwinner domain.Due to our choice of focus, we do not include properties based on the Condorcet principle, such as, e.g., the stability of Barberà and Coelho [4].We stress that, since multiwinner rules have a very diverse range of applications, our properties should not necessarily be understood in the normative way: the desirability of a particular property can only be evaluated in the context of a specific application.Throughout this section, we write R to denote some given multiwinner rule.
Our first axiom is nonimposition.It requires that each size-k set of candidates can win.This is a basic requirement that is trivially satisfied by all rules that we consider.

Nonimposition. For each set of candidates C and each
The next three axioms-consistency, homogeneity, and monotonicity-are adapted from the single-winner setting.For the first two, the adaptation is straightforward.
Consistency.For every pair of elections Homogeneity.
We now consider monotonicity.If c belongs to a winning committee W then, generally speaking, we cannot expect W to remain winning when c is moved forward in some vote.For example, this shift may hurt other members of W . Indeed, none of our rules satisfies this strict version of monotonicity.However, there are two natural relaxations of this condition.One option is to require that after the shift c belongs to some winning committee.Alternatively, we may restrict forward movements of c, prohibiting it to overtake other members of W . (We point the reader to the work of Sanver and Zwicker [44] for an extensive discussion of monotonicity in the context of irresolute voting rules.) ), then for each E ′ obtained from E by shifting c one position forward in some vote v it holds that: (1) for candidate monotonicity: c ∈ W ′ for some for non-crossing monotonicity: if c was ranked immediately below some b ∈ W , then W ∈ R(E ′ , k).
Our next axiom, committee monotonicity, is specific to multiwinner elections, as it deals with changing the size of the desired committee.Intuitively, it requires that when we increase the size of the target committee, none of the already selected candidates should be dropped.Our phrasing is somewhat involved because R returns sets of committees.
Committee Monotonicity.For each election E = (C, V ) the following conditions hold: (1) The second condition in the definition above is aimed to prevent the following situation.Consider an election E with candidate set C = {a, b, c, . ..}.Without condition (2) a committee-monotone rule R would be allowed to output the following winning committees: R(E, 1) = {{a}}, R(E, 2) = {{a, b}, {b, c}}, and so on.Note that the committee {b, c}, which suddenly appears in R(E, 2), breaks what we would intuitively think of as committee monotonicity, but is not ruled out by condition (1) alone.
Other authors already discussed committee monotonicity in the context of multiwinner voting, but under different names and usually by arguing that it is paradoxical that a given voting rule fails committee monotonicity [41,47].In Section 5 we discuss committee monotonicity in detail, argue that failing it may be seen as a positive feature of a multiwinner rule, and use it to axiomatize best-k rules.
The final three axioms represent three implementations of Dummett's condition known as proportionality for solid coalitions [20].Dummett's original proposal is as follows: Consider an election with n voters where the goal is to pick k candidates.If for some ℓ ∈ [k] there is a group of ℓn k voters that all rank the same ℓ candidates on top, these ℓ candidates should be in a winning committee.This requirement, which tries to capture the idea of proportional representation, seems to be quite strong.We are not aware of a single rule that satisfies it. 2The following three axioms are weaker and reflect the same idea.
Solid Coalitions.For each election E = (C, V ) and each k ∈ [ C ], if at least V k voters rank some candidate c first then c belongs to every committee in R(E, k).
, such that each voter ranks some member of W first and each member of W is ranked first by either We remind the reader that we list only the axioms that make sense for preference-based rules which are in some broad sense close to scoring rules.There are however preferencebased axioms that are geared towards Condorcet principle.The following axiom is an example [19], though it can also be seen as a generalization of the unanimity property.

Fixed Majority. For each election
, such that a strict majority of voters rank all member of W above all non-members of W , then R(E, k) = {W }.
Almost all the rules that we consider in this paper fail to satisfy this axiom.For most of them, this already happens in the single-winner setting, with k = 1.Indeed, Plurality is the only single-winner scoring protocol that guarantees that a candidate ranked on top by a majority of the voters is the unique winner.However, quite interestingly, Bloc does satisfy the fixed majority property. 3This is yet another reason to view Bloc as a rule of a different kind than separable committee scoring rules (even though at the level of formal definition Bloc is very close to separable rules; see the discussion in Section 3.2).

Committee Monotonicity
The desirability of committee monotonicity depends strongly on the application: if we are choosing finalists of a competition, then it is imperative to use a rule that has this property, but in the context of proportional representation requiring that the rule is committee monotonic may prevent selecting a truly representative committee.Indeed, this was already suggested by Black [8]: Consider a society with single-peaked preferences regarding the left-right political spectrum.If we are picking a single candidate (i.e., if k = 1) then it is most natural to select as centrist a candidate as possible (formally, we would select a Condorcet winner).However, if we were to select two candidates to represent the society (i.e., if k = 2), then selecting a "moderate left-wing" and a "moderate right-wing" candidate would, intuitively, give more proportional representation than selecting the centrist candidate and an additional one (if the additional candidate were to the left of the centrist one, the right-wing voters would be neglected; if the candidate were to the right of the centrist one, the left-wing voters would be neglected).
The above intuitions are further strengthened by the fact that committee monotonicity axiomatically characterizes the class of best-k rules.
Theorem 3. A k-committee selection rule is committee monotonic if and only if this rule is a best-k rule.
Proof.Let R be a best-k rule and let F be the underlying social preference function.

Consider an election
A similar argument shows that R satisfies the second committee-monotonicity condition.
Conversely, assume that R satisfies committee monotonicity.We will show that it is a best-k rule by revealing the underlying social preference function F .Let E = (C, V ) be some election where C = {c 1 , . . ., c m }.We define F (E) to contain all linear orders ≻ that satisfy the following condition: If π is a permutation of [m] and Using the two conditions from the definition of committee monotonicity, it is easy to verify that F indeed defines R.
Thus SNTV, k-Borda, and all separable committee scoring rules satisfy committee monotonicity.On the other hand, Greedy-CC satisfies committee monotonicity and, in effect, is a best-k rule as well.
Proof.Given an election E and an integer k, Greedy-CC performs k iterations, in each picking one member of the committee.However, the member of the committee picked in each iteration depends only on which members were chosen previously and not on k.Thus, after Greedy-CC performs k iterations to compute committee of size k, one can simply perform one more iteration to obtain committee of size k + 1. Conversely, if Greedy-CC is to compute a committee of size k + 1, it first computes a committee of size k.In effect, it is clear that Greedy-CC is committee monotone.
For separable committee scoring rules, by Theorem 1 their underlying social welfare functions are the scoring ones.This possibly distinguishes them from rules such as Greedy-CC, whose underlying social preference functions may not be based on scoring rules.
It is easy to see that for k = 1 the unique winning committee is {c} but for k = 2 the unique winning committee is {a, b}.
The case of Bloc was resolved by Staring [47], but we include a simple argument for the sake of completeness.We use election E = (C, V ) with C = {a, b, c} and four votes: For k = 1 Bloc has unique winning committee {a}, but with k = 2 the unique winning committee is {b, c}.
Our example for STV is a bit more involved.Consider E = (C, V ) with C = {a, b, c, d} and with 24 voters.We have: For k = 1, the Droop quota is ⌊ 24  2 ⌋ + 1 = 13.In the first round no candidate meets the quota and so STV eliminates the candidate with the lowest plurality score, that is, b.In the next round still no candidate meets the quota and STV eliminates d.In the resulting profile c has plurality score 13 and is the unique winner for k = 1.
For k = 2, the Droop quota is ⌊ 24 3 ⌋ + 1 = 9.Thus in the first round STV picks a and removes it from the election together with 9 voters that rank it first.The remaining two voters that supported a transfer their votes to b, who now has plurality score 5. Thus in the next round no candidate meets the quota and STV eliminates c.In the next round d has plurality score 10 and is selected.The unique winning committee is {a, d}.Corollary 6. STV, Bloc, ℓ 1 -CC, ℓ min -CC, ℓ 1 -Monroe, ℓ min -Monroe, and Greedy-Monroe are not best-k rules.

Dummett's Proportionality
Properties in the spirit of Dummett's proportionality condition (with the exception of unanimity) are geared toward rules that aim to achieve proportional representation of the voters.Thus, in this section, we judge multiwinner rules from this perspective.
We start by considering the solid coalitions property.It is easy to see that it is satisfied by both SNTV and STV.
Theorem 7. SNTV has the solid coalitions property.STV also has it for each election with n ≥ k(k + 1), where n is the number of voters and k is the size of the winning committee.
Proof.For SNTV it suffices to note that if there are n voters and some candidate c is ranked first by n k voters, then-by a simple counting argument-there cannot be k other candidates each of whom is ranked first by at least n k voters.Thus c must be included in each winning committee.
For STV, the Droop quota has value ⌊ n k+1 ⌋ + 1 and, if n ≥ k(k + 1), then it is smaller or equal to n k .Thus if there is a candidate c who is ranked first by n k candidates then this candidate will be included in the winning committee.
On the other hand, even though this property seems to be very much in spirit of the Monroe and Chamberlin-Courant rules, ℓ 1 -Monroe, ℓ 1 -CC, ℓ min -Monroe, and ℓ min -CC fail to satisfy it.Yet, it is satisfied by Greedy-Monroe.
It can be easily verified that none of the four versions of the Chamberlin-Courant and Monroe rules elect a committee of size 3 that contains d, even though this would be required by the solid coalitions property.Indeed, for the committee {a, b, c} the total satisfaction is 9 C − 12 in the utilitarian version and C − 1 in the egalitarian one.However if d is on the committee, then at most two of the candidates among a, b, c, e, f, g are also in the committee and so the total satisfaction is, respectively, at most 9 C − 13 and C − 2.
For the second part of the theorem, we consider Greedy-Monroe.Take some election with n voters, where we seek a committee of size k.Suppose that some candidate c is ranked first by at least n k voters.Greedy-Monroe starts by picking candidates ranked first by at least n k voters.By the time it considers c, each of the voters that rank c first remains unassigned, so it picks c.
We believe that the solid coalitions property is desirable, but not crucial for applications that require proportional representation (e.g., parliamentary elections).In contrast, the consensus committee property, which we discuss next, seems to be fundamental.Indeed, it is satisfied by almost all rules that aim to achieve proportional representation.
When k is a divisor of n, the consensus committee property is satisfied by every rule that has the solid coalitions property.In particular, it is satisfied by SNTV, STV (if there are sufficiently many voters) and Greedy-Monroe.It is also satisfied by ℓ 1 -CC, ℓ min -CC, ℓ 1 -Monroe, and ℓ min -Monroe, but, interestingly, not by Greedy-CC.This reveals a major deficiency of the latter rule: It makes decisions regarding the inclusion of some candidate c into the committee based on the preferences of the voters to whom c would not be assigned.This is very problematic for a rule that seeks to approximate ℓ 1 -CC; interestingly, the feature of Greedy-CC that causes this is also responsible for the rule being committee monotone.Proposition 9. Bloc, k-Borda and Greedy-CC do not have the consensus committee property (nor the solid coalitions property).
Proof.Consider an election with C = {a, b, c, d} and two voters with preference orders b ≻ c ≻ d ≻ a and a ≻ c ≻ d ≻ b.We seek a committee of size k = 2. Then the consensus committee is {a, b} but each of these rules includes c in each winning committee and thus fails the consensus committee property.
For SNTV, ℓ 1 -CC, k-Borda and Bloc, the above results can also be seen as incarnations of the following two more general results regarding committee scoring rules.
Proof.We first consider the case when 0 < γ(1) ≤ kγ (2).Consider an election E = (C, V ) with m candidates and k + 1 voters.Let c 1 , . . ., c k , x be some k + 1 candidates from C. We form the preference orders of the voters so that each c i , 1 ≤ i ≤ k − 1, is ranked first by exactly one voter, c k is ranked first by two voters, the voters that do not rank c 1 first rank c 1 last, and each voter ranks x second.Aside from that, the preference orders are arbitrary.If R had the consensus committee property, then {c 1 , . . ., c k } would be the unique winning committee.Since R by Theorem 1 is a best-k rule for scoring vector s = (s 1 , . . ., s m ), with m = C , where s i = γ(i), it suffices to show that c 1 gets lower score than x with respect to s.Indeed, the score of x is (k + 1)s 2 and the score of c 1 is s 1 .Since we assume that 0 < γ(1) ≤ kγ(2), we have s 1 ≤ ks 2 .Moreover, since each c 2 , . . ., c k has s-score equal or higher than c 1 , we have that {c 1 , . . ., c k } is not a winning committee.Thus R does not have the consensus committee property.
Suppose now that γ(1) > kγ (2).Consider an arbitrary election E = (C, V ), where there is a group W of k candidates such that each voter ranks some member of W first and each member of W is ranked first by either ⌊ n k ⌋ or ⌈ n k ⌉ voters.Consider a candidate x / ∈ W .Its s-score (where s is defined as in the previous paragraph) is at most ns 2 .On the other hand, the score of each candidate in W is at least ⌊ n k ⌋s 1 .By assumption, ⌊ n k ⌋s 1 > ns 2 for sufficiently large n and, thus, R satisfies the consensus committee property for such n.
Proof.Consider an election E that satisfies the conditions of the consensus committee property.If γ(1) > γ(2) then clearly the k candidates from E that are ranked first form a unique winning committee.On the other hand, if γ(1) = γ(2) then it is possible to form an election with more than one winning committee.It suffices to consider a profile of k votes over 2k candidates, where each vote ranks a distinct pair of candidates on top.In this case, there are at least 2 k different winning committees.
Our final instantiation of Dummett's proportionality for solid coalitions is the unanimity property.Every committee scoring rule satisfies its weak variant.
Theorem 12. Every committee scoring rule R satisfies weak unanimity.
Proof.Consider an election E = (C, V ) where every voters ranks candidates from some set W , W = k, on top.Let f be the committee scoring function for R. By definition, for each voter v in V and each size-k set Q of candidates, we have f It is immediate that ℓ 1 -Monroe, ℓ min -Monroe, Greedy-Monroe, Bloc and k-Borda satisfy strong unanimity, and that SNTV, ℓ 1 -CC, ℓ min -CC, and Greedy-CC do not (to see this, consider an election where all the voters have the same preference order; by definition, in those cases these rules choose all the committees that include the candidate ranked first by all the voters).
Finally, we note that STV satisfies strong unanimity.If there is some set W of k candidates that each of the n voters ranks on top, then in every round of STV there is a candidate from W that is ranked first by at least ⌊ n k+1 ⌋ + 1 voters.

Monotonicity
Being monotonic is a natural and easily satisfiable condition for single-winner rules.Among the few examples of prominent non-monotonic single-winner rules are STV and Dodgson's rule [12].In contrast, for multiwinner rules monotonicity is a rather demanding property.However, all committee scoring rules satisfy candidate monotonicity, and all separable committee scoring rules satisfy non-crossing monotonicity.
Theorem 13.Let R be a k-committee scoring rule.Then R satisfies candidate monotonicity.
Proof.Consider an election E = (C, V ) with C = m.Let f be the k-committee scoring function defining R for m candidates and k winners.Let W be a committee in R(E, k) and let c be a candidate in W . Consider a vote v ∈ V that does not rank c first, and replace it with a vote v ′ obtained from v by shifting c one position forward.Denote the resulting election by E ′ .By construction, we have f (pos v ′ (W )) ≥ f (pos v (W )).On the other hand, for each committee S ⊆ C \ {c}, we have f (pos v ′ (S)) ≤ f (pos v (S)).Since W was a winning committee for E, this means that either W is also a winning committee for E ′ or some committee W ′ ∈ R(E ′ , k) has a higher score.We must have c ∈ W ′ , since only committees with c can have a higher score in E ′ , compared to E. Theorem 14.Let R be a weakly separable k-committee scoring rule.Then R satisfies non-crossing monotonicity.
Proof.Let E = (C, V ) be an election with m candidates.Let f be a weakly separable committee scoring function defining R. Let W be a winning committee and let c ∈ W . Consider a vote v where c is not directly preceded by a member of W .Let d be the candidate who is directly above c in v (d / ∈ W ) and let v ′ be a vote obtained from v by swapping c and d.
After the swap, committees that include c and not d gain the same number f (pos v ′ (W ))− f (pos v (W )) of points and this number is non-negative; every committee with both c and d (or with neither c nor d) maintains the same score, and every committee with To complete the discussion of committee scoring rules, we need to consider non-separable committee scoring rules with respect to the non-crossing monotonicity.It appears that these rules (such as ℓ 1 -CC) do not normally satisfy the non-crossing monotonicity.
Proof.Consider election E = (C, V ) where C = {a, b, c, d, x 1 , . . ., x 6 } and V = (v 1 , . . ., v 6 ).The preference orders of the voters are as follows: The reader can quickly verify that if we seek a committee of size k = 2 then, according to ℓ 1 -CC, there are three winning committees, {a, b}, {a, c}, and {b, c}, each with satisfaction 6 C − 11.If we focus on committee {a, c}, non-crossing monotonicity requires this committee to still be winning after shifting c forward by one position in v 1 .However, if that happens, the satisfaction of {a, c} does not change but the satisfaction of {b, c} increases to 6 C − 10.The same construction works for ℓ 1 -Monroe, Greedy-CC, and Greedy-Monroe.
While ℓ min -CC is not a committee scoring rule, it behaves similarly to non-separable committee scoring rules.However, this is not the case for ℓ min -Monroe.
Proof.The proof of the first part is similar to that of Theorem 13.If W is a winning committee for ℓ min -CC and c is a candidate in W , then shifting c forward has the following effect.The aggregate satisfaction of every committee that includes c either increases by one, or stays the same.The aggregate satisfaction of every committee not containing c either stays the same or decreases by one.Thus c belongs to at least one winning committee.To show that ℓ min -Monroe fails candidate monotonicity, consider an election with candidate set C = {a, b, c, d} and four voters V = (v 1 , v 2 , v 3 , v 4 , v 5 , v 6 ), with preference orders as follows: For k = 2, the winning committees are {a, b} and {c, d}, both with satisfaction C − 2. If we shift a forward by one position in v 4 , the satisfaction of {a, b} decreases to C − 3 but the satisfaction of {c, d} stays the same.
To see that ℓ min -CC fails non-crossing monotonicity, consider election , where the voters have the following preference orders: We seek a committee of size k = 2.There are four tied ones, {b, c} and {a, c}, {x 1 , c} and {x 2 , c}, all with aggregate satisfaction C − 4.These are the winners.Non-crossing monotonicity requires that if we shift c forward in v 2 , then {b, c} should still be a winning committee.However, this committee's satisfaction stays the same, whereas the satisfaction of {a, c} increases to C − 3. The same construction applies to ℓ min -Monroe.
The remaining multiwinner rules studied in this paper fail each of our monotonicity criteria.For STV this is well-known to happen even for k = 1.For the rest of the rules, we provide the following result.
Proof.First, we note that the argument given in Theorem 16 to show that ℓ min -Monroe is not candidate monotone (profile (2) and moving a forward in vote v 4 ) in fact also applies to ℓ 1 -Monroe.
We move on to Greedy-Monroe.Let E = (C, V ) be an election with C = {a, b, c, d} and V = (v 1 , . . ., v 8 ) whose preference orders are: For k = 2, under Greedy-Monroe there are two winning committees {a, c} and {b, d}.This is so because in the first iteration, Greedy-Monroe picks either a or b.If it picks a, then in the next iteration it picks c.If it picks b, then irrespective which voters it chooses to assign to b in the first iteration (among the assignments allowed under Greedy-Monroe), in the second iteration it picks d.However, if we shift c forward by one position in v 6 then only {b, d} remains winning (Greedy-Monroe no longer can pick a in the first iteration).This shows that Greedy-Monroe is not candidate monotone.
For the case of Greedy-CC, consider the election E = (C, V ) with C = {a, b, c, d} and V = (v 1 , . . ., v 6 ), where For k = 2, Greedy-CC declares {a, b} and {a, c} as winners.(In the first iteration Greedy-CC picks either a or b.In the former case, in the second iteration it picks either b or c.In the latter case, i.e., if it picks b in the first iteration, in the second iteration it picks c).
However, if we shift c forward by one position in v 6 , then {a, b} and {a, d} are winning (in this case, in the first iteration Greedy-CC has to pick b, and then in the second iteration there is a tie between a and d).Thus Greedy-CC fails candidate monotonicity as well.

Consistency and Homogeneity
For single-winner rules, the famous Young's theorem [50] says that only scoring rules and their compositions satisfy consistency.While we do not know how to extend this result to multiwinner rules, the situation seems to be similar: We show that every committee scoring rule satisfies consistency, whereas other rules fail it.
Proposition 18.Every committee scoring rule satisfies consistency.In particular, ℓ 1 -CC is consistent.
Proof.Let R be a k-committee selection scoring rule and let f be the corresponding committee scoring function.Let E 1 and E 2 be two elections, , where C = m, and where ).If it were not the case, then there would have to be a committee Q of size k such that either the score of Q in E 1 or the score of Q in E 2 were higher than that of W in these elections.This contradicts the choice of W .
Similarly, if T is a winning committee for E 1 + E 2 then it must be the case that T ∈ R(E 1 , k) and T ∈ R(E 2 , k).This is so, because the score of T in E 1 + E 2 is the same as that of W .However, if T were not in R(E 1 , k), then this would mean that the score of T in E 1 is smaller than the score of W in E 1 and, so, the score of T in E 2 is higher than that of W in E 2 .This would mean that it is not the case that W ∈ R(E 1 , k) and W ∈ R(E 2 , k).Now let us turn our attention to the negative results.We note that some of our rules, such as STV, ℓ min -CC and ℓ min -Monroe, can be ruled out to be consistent on the basis of Young's result because for k = 1 these rules are not scoring rules.
On the other hand, for k = 1 each of ℓ 1 -Monroe, Greedy-CC and Greedy-Monroe is equivalent to Borda, which is consistent.For these rules, we directly show that they are not consistent.
The voters in V 2 have the following preferences: We seek a committee of size k = 2.It is straightforward to check that under ℓ 1 -Monroe, committee {a, c} would win in both E 1 and E 2 but in E 1 + E 2 , committee {a, b} has higher satisfaction than {a, c}.
For Greedy-Monroe, we use C = {a, b, c, d} and elections ) and E 2 = (C, V 2 ), where V 1 and V 2 have two voters each.The voters in V 1 have the following preferences: The voters in V 2 have the following preference orders: For k = 2, Greedy-Monroe chooses {a, b} as the winning committee in both elections, but for E 1 + E 2 it chooses {a, b} and {a, c} (due to the parallel-universe tie-breaking).This shows that Greedy-Monroe is not consistent.
For Greedy-CC we, again, take C = {a, b, c, d} and elections ) and E 2 = (C, V 2 ) but this time V 1 and V 2 contain four voters each.The voters in V 1 have the following preference orders: The voters in V 2 have the following preference orders: For k = 2, in both E 1 and E 2 , Greedy-CC picks {a, b}.However, in E 1 + E 2 it picks {a, c} and {b, c} (to see that Greedy-CC is non consistent it suffices to note that in E 1 + E 2 it has to pick c in the first iteration because c is the unique Borda winner in this election).
We now consider homogeneity.Naturally, committee scoring rules are homogeneous because consistency implies homogeneity.For other rules, the situation is more complex.Theorem 21.Both ℓ min -CC and Greedy-CC satisfy homogeneity.
We omit these proofs since they are straightforward.Interestingly, neither of the variants of Monroe's rule is homogeneous, although, as we will see in Theorem 23, ℓ 1 -Monroe, ℓ min -Monroe are partially homogeneous.
Proof.Consider an election with candidate set C = {a, b, c, d} and three voters v 1 , v 2 , and v 3 , with the following preference orders: We seek a committee of size k = 2.For ℓ 1 -Monroe, Greedy-Monroe and ℓ min -Monroe, the unique winning committee of size 2 is {a, c}.However, for 2E the winner sets for these rules include {a, b}.
On the positive side, if the number of voters is divisible by the size of the committee, then ℓ 1 -Monroe and ℓ min -Monroe are homogeneous.In essence, this means that variants of Monroe fail homogeneity due to rounding problems in the Monroe criterion.One solution would be to clone each voter k times when seeking a committee of size k.We do not consider this modification of Monroe's rule here, but it would be interesting to see how the satisfaction of a committee elected in this way compares to that elected without cloning.
Theorem 23.Both ℓ 1 -Monroe and ℓ min -Monroe satisfy homogeneity, provided that the number of voters n in the election is divisible by the size k of the committee to be selected.
Proof.Let R ∈ {ℓ 1 -Monroe, ℓ min -Monroe}, pick an election E = (C, V ), and let k be a positive integer that divides n = V .We will show that R(E, k) = R(tE, k) for each t > 0.
Let W be a committee that wins in tE.We refer to the members of W as the winners.Let Φ : tV → C be an assignment of candidates to voters witnessing that W ∈ R(tE, k).By Monroe's criterion, for each w ∈ W we have Φ −1 (w) = nt k .We now proceed as follows.First, we show how to transform Φ into an assignment Φ ′ such that (a) under Φ ′ each winner represents exactly n k voters in each copy of V and (b) the satisfaction of the voters under Φ ′ is the same as under Φ.We then prove that Φ ′ can be further transformed into Φ ′′ that uses the same assignment for each copy of V .
Let V 1 , . . ., V t be t copies of V so that tV we assume that within each V i , i ∈ [t], voters are listed in the same order.For each i ∈ [t], ℓ ∈ [n], we write v i,ℓ to denote the ℓ-th voter in V i .Our proof relies on the observation that for each ℓ ∈ [n] and each i, j ∈ [t], if we take some assignment function Φ and swap its values for v i,ℓ and v j,ℓ , then we get an assignment function Φ ′ with the same societal satisfaction.To exploit this fact, for each ℓ ∈ [n], we write rep(ℓ) = {Φ(v i,ℓ ) | i ∈ [t]} to denote the set of the representatives (under Φ) of the set of ℓ'th voters in the profiles V 1 , V 2 , . . ., V t .
We now show that, using the transformations just described, it is possible to transform Φ into an assignment Φ (1) which assigns each winner from W to exactly n k voters from V 1 .We use the fact that for each ℓ ∈ [n], we can assign an arbitrary member of rep(ℓ) to voter v 1,ℓ by swapping the representative of v 1,ℓ with the representative of v j,ℓ , for appropriately selected j.We build the following bipartite graph (the vertices are partitioned into the set of voter vertices and the set of winner vertices): 1.The voter vertices are exactly the voters from V 1 .
2. For each winner w ∈ W , we have n k winner vertices w 1 , . . ., w n k which we will call clones of w.It is clear that if there is a perfect matching between the voter vertices and the winner vertices in this graph, then it is possible to transform Φ into Φ (1) that assigns each winner to exactly n k voters in V 1 .Given such a perfect matching, we transform Φ so that for each voter v 1,ℓ that is matched to some vertex w u , we swap this voter's representative (if it is not already w) with the representative of some voter v j,ℓ for whom Φ(v j,ℓ ) = w.This is possible since w ∈ rep(ℓ).Thus it remains to show that our graph has a perfect matching.To this end, we use the famous Hall's theorem.
For each subset V ′ of voter vertices, we define the set of neighbors of V ′ , denoted N (V ′ ), to be the set of those winner vertices that are connected to some member of V ′ .Hall's theorem says that there is a perfect matching in our graph if and only if for every set V ′ of voter vertices we have Let V ′ be some arbitrary subset of voter vertices.By the construction of our graph, we know that if N (V ′ ) contains one clone of w, then it contains all of them, hence N (V ′ ) is of the form q n k , where q is a positive integer, and that N (V ′ ) corresponds to a set of q winners, each of whom has n k clones in N (V ′ ).This means that in tV there is a group of t V ′ voters represented under Φ by q winners.Since in tV each winner is assigned to exactly t n k voters, it must be that t V ′ ≤ qt n k .This is equivalent to V ′ ≤ q n k = N (V ′ ) , the requirement of the Hall's theorem.Thus there is a perfect matching for our graph and the desired assignment Φ (1) exists.Now, removing V 1 from consideration and repeating the above procedure for V 2 , then V 3 , and so on, we can eventually transform Φ (1) into Φ ′ , where each winner represents exactly n k voters from each sequence of voters V i , i ∈ [t].It is then easy to see that we can replace Φ ′ with Φ ′′ that uses the same assignment of winners to voters within each copy of V i (each V i has an assignment with the same societal satisfaction; otherwise the original assignment Φ would not have been optimal).This implies that W ∈ R(E, k) because otherwise there would be some set W ′ ∈ R(E, k) that would give lower satisfaction to the voters in tE than W gives. Analogously, this means that every W ′ ∈ R(E, k) also belongs to R(tE, k).
Proposition 24.Greedy-Monroe fails homogeneity even if the size of the committee divides the number of voters.
Proof.Let E = (C, V ) be an election with C = {a, b, c, d} and V = {v 1 , . . ., v 6 }.The voters have the following preference orders: We seek a committee of size k = 2.Under Greedy-Monroe, the tied-for-winning committees are {a, b} and {a, c}.Indeed, in the first iteration Greedy-Monroe may pick a.It can assign ). Depending on the choice, in the second iteration it picks either c or b.In the first iteration it can also pick b, then in the second iteration it picks a.Now consider election 2E.For each i ∈ {1, . . ., 6}, j ∈ {1, 2} we write v j i to refer to v i in the j-th copy of E. In the first iteration, Greedy-Monroe again is allowed to pick a (due to parallel-universes tie-breaking).By parallel-universes tie-breaking, it is allowed to assign Thus, in the second iteration, the remaining votes are: The unique Borda winner of this election is d, so Greedy-Monroe picks d in the second iteration.This means that {a, d} is a winning committee in 2E, a contradiction.
The above proposition relies heavily on parallel-universes tie-breaking.It is possible to refine the intermediate tie-breaking procedure of Greedy-Monroe so that it becomes homogeneous when k divides V .We omit the details here.

Related Literature
With all the preceding discussions in hand, it is high time to discuss how our results and approaches compare to others in the literature.Unfortunately, the literature on the properties of multiwinner rules is still relatively sparse, compared to that regarding single-winner rules, and is scattered between different fields of research, ranging from behavioral science, through political science, social choice theory, to computer science.Here we review those papers that are in spirit closest to our work.
The paper most closely related to ours is that of Felsenthal and Maoz [23].They consider four k-choice functions, the Plurality rule (i.e., in our terminology, the SNTV rule), the Approval rule,4 the Borda rule (i.e., k-Borda), and STV.They adapt a range of single-winner normative properties to the multiwinner setting and study them in the context of these rules.The main two differences between our paper and theirs are as follows.First, we study a somewhat different set of rules (in particular we do not consider the Approval rule, but we do consider Bloc and a number of rules focused on proportional representation).Second, the set of axioms that we consider is closer in spirit to the goal of achieving proportional representation than theirs.On the one hand, we introduce some new axioms that try to capture proportional representation (such as consensus committee and solid coalitions properties), and, on the other hand, we do not consider axioms related to the Condorcet principle (and Felsenthal and Maoz do).Naturally, our papers also have many similarities.Both we and Felsenthal and Maoz consider monotonicity, though our view is slightly more detailed (we study two variants, candidate monotonicity and non-crossing monotonicity).Both us and Felsenthal and Maoz [23] study committee monotonicity, which they call continuity.We prefer our name since continuity in social choice refers to a different property [50].Finally, both Felsenthal and Maoz and us study the consistency property, which is crucial when one considers rules based on scoring protocols (indeed, as shown by Young [50], up to some tie-breaking specifics, scoring protocols are the only single-winner rules that are consistent, and, thus, it is natural to expect that a similar result holds in the multiwinner setting; very interestingly, consistency also plays an important role in Bock et al.'s study of consensus-based multiwinner rules [9]).
Young's consistency-based characterization of scoring rules, as well as his characterization of the Borda rule, have already inspired researchers working on multiwinner rules.For example, Debord extended Young's characterization of Borda to the case of k-Borda, by showing that it is the rule that satisfies the axioms of neutrality, faithfulness, consistency, and the cancellation property (we point readers to his work for details on these properties [18]).In his follow-up work, Debord also introduced some new axioms for multiwinner rules.These axioms, however, are geared toward the case of rules that elect what he calls k-elites [19], which are multiwinner analogs of Condorcet winners.Indeed, there is a number of multiwinner rules that focus on electing a committee by following the principle of Condorcet consistency; related issues were considered, for example, by Barberà and Coelho [4], Kaymak and Sanver [30] Fishburn [26], Ratliff [41], and others.Since we do not consider Condorcet rules, we do not present this literature in depth.
The study of committee selection rules in approval voting framework is well-advanced.Kilgour [32] describes a number of approval-based voting rules that elect a committee of fixed size and proved some of their basic properties, Kilgour and Marshall [33] give a nice survey of approval-based committee selection rules and add some new ones.Aziz et al [2] have recently introduced an axiom called Justified Representation and showed that in their framework the only committee selection rule that satisfies it is Proportional Approval Voting (PAV).Again, we do not focus on approval-based rules so we point the readers to the abovecited papers for more detailed discussions.
Multiwinner voting often raises controversies.For example, Staring [47] demonstrates that Bloc rule is extremely committee non-monotonic (he calls it increasing-committee-size paradox).He demonstrates a profile where the winning committee of size three is disjoint from the winning committee of size two, and the winning committee of size four is disjoint from those for sizes two and three.Similar phenomena is observed by Ratliff [41] for other rules, who even titled his paper "Some Startling Inconsistencies when Electing Committees" (we mention that, in another paper, he also points to some possibly undesirable behavior in other multiwinner settings [42]).On the other hand, in this paper we view the fact that a rule fails to be committee monotonic as a rather expected feature of every rule that attempts to achieve proportional representation of the voters.This is yet another reason to believe that multiwinner rules can only be judged in the context of their applications.A rule for selecting a parliament should satisfy quite different desiderata than a rule used for shortlisting.
Multiwinner voting rules are more complex than the single-winner ones also from the computational standpoint.While there are some computationally hard single-winner rules (most notably the rules of Kemeny, Young, and Dodgson [5,6,28,29,43]), for the case of multiwinner voting, this problem seems to be much more pronounced.For example, Darmann [17] showed that testing if a given committee satisfies a particular Condorcet criterion is NP-hard.Similarly, it is NP-hard to compute winners under Monroe's rule [7,40], Chamberlin-Courant's rule [7,37,40], and many other similar rules [45].The same holds for a number of approval-based rules such as PAV [3,45] or Minimax Approval rule [36] of Brams et al. [11], or even for STV with parallel-universes tie-breaking [16].In effect, many researchers have sought efficient approximate algorithms for these rules, many of which can be viewed as full-fledged voting rules in their own right.Indeed, this is how the Greedy-CC [37] and Greedy-Monroe [46] rules that we study came to be.Nonetheless, there is also a number of natural multiwinner rules that are polynomial-time computable.These include, for example, rules based on electing candidates with the k highest scores according to some scoring protocol (e.g., SNTV, Bloc, k-Borda), but also some other, more complicated rules (see, e.g., the work of Klamler et al. [34]).

Conclusions
We formalized a number of natural properties for multiwinner voting rules and conducted a comprehensive comparison of ten prominent multiwinner rules with respect to these properties.The choice of these rules was guided by the fact that each of them, in some broad sense, is based on some single-winner scoring rule.In the course of our study, we identified two natural families of multiwinner rules, best-k rules and committee scoring rules.The latter class is particularly interesting because it contains both rules of low complexity, such as k-Borda, SNTV, and Bloc, and rules with hard winner-determination problems, such as ℓ 1 -CC.We have put forward a framework for studying multiwinner rules and considered a number of their properties.We believe that our results give a better understanding of applicability of various multiwinner rules to particular tasks.For example, we see that best-k rules are well-suited for picking a group of finalists in a competition, whereas rules based on the Monroe criterion (ℓ 1 -Monroe, ℓ min -Monroe, and Greedy-Monroe), as well as STV, seem to be more appropriate for applications that require proportional representation (e.g., parliamentary elections).In this context, Greedy-Monroe is particularly interesting.It was derived as an approximation algorithm for ℓ 1 -Monroe [46], but it has more appealing properties than the original rule.We believe that Greedy-Monroe should be taken as a full-fledged voting rule.
Our results for ℓ 1 -CC and ℓ min -CC are similar to those for Monroe, but intuitively these rules are better suited for applications such as movie selection (see the introduction) than, say, parliamentary elections.The reason is that they may assign very different numbers of voters to each winning candidate (naturally, one could imagine rules for parliamentary elections where voters would be represented by more than a single person-and thus different winning candidates might represent different numbers of voters-but ℓ 1 -CC and ℓ min -CC do not operate on such basis).Thus, if ℓ 1 -CC or ℓ min -CC were used to elect a parliament, this parliament would have to use weighted voting in its operation.A recommendation system, on the other hand, simply needs to present users with a "committee" of items so that as many customers (voters) as possible would find at least one of them satisfying.
In this light, it is a bit disappointing that Greedy-CC, which was designed as an approximation algorithm for ℓ 1 -CC, does not seem to perform very well in our comparison.Indeed, it fails to satisfy the solid coalitions property (like ℓ 1 -CC but unlike Greedy-Monroe) and it fails to satisfy the consensus committee property (unlike every other rule that focuses on some form of proportional representation).This latter fact can be seen as a consequence of Greedy-CC satisfying committee monotonicity (which we argued to be undesirable from the point of view of proportional representation).
After comparing Greedy-CC and Greedy-Monroe, it is tempting to simply use Greedy-Monroe in Greedy-CC's place.However, perhaps a better idea would be to modify Greedy-Monroe to not respect the Monroe's criterion.Then the challenge would be to optimally pick the numbers of voters that Greedy-Monroe considers in each iteration.We believe that studying such variant of Greedy-Monroe is an important research direction.
Finally, our results regarding SNTV are quite interesting.It turns out that, among all the rules that we consider, SNTV satisfies all the properties that we defined (though it only satisfies unanimity in the weak sense).One explanation for this fact is that it can be viewed as an approximation of the rule that simply picks all the candidates that are ranked first by at least one voter, and, in this sense, is close to implementing direct democracy.Yet one should not forget that SNTV ignores each voter's preferences beyond the top candidate and, thus, inherits all negative features of the Plurality rule.

Theorem 8 .
ℓ 1 -CC, ℓ min -CC, ℓ 1 -Monroe, and ℓ min -Monroe do not have the solid coalitions property, but Greedy-Monroe does have it.Proof.Let us consider an election with candidate set C = {a, b, c, d, e, f, g} and nine voters whose preference orders are

3 .
For each voter vertex v 1,ℓ ∈ V 1 and each winner w ∈ W , there are edges between v 1,ℓ and each of the winner vertices w 1 , . . ., w n k , if w ∈ rep(ℓ), and no such edges exist, if w / ∈ rep(ℓ).
Plurality score.The plurality score of a candidate c is the number of voters that rank c first.
t-approval score.Let t be a positive integer.The t-approval score of a candidate c is the number of voters that rank c among the top t positions.Borda score.Let v be a vote over a candidate set C. The Borda score of a candidate

Table 1 :
Summary of results.
√ and × indicate that the rule has/does not have the respective property.C means candidate monotonicity and NC means non-crossing monotonicity (C/NC means satisfying both conditions)."Fix maj." in the Unanimity column for Bloc means that not only Bloc is unanimous in the strong sense, but also that it satisfies the fixed majority property, which is stronger (none of the other rules satisfy it).The properties marked with (⋄) hold for STV when n ≥ k(k + 1); property marked with (♥) requires STV to use non-rounded Droop quota and fractional votes.Properties marked with (♣) hold if n is divisible by k and (♠) in addition requires a specific intermediate tie-breaking rule.