1 Algorithms for Large Data Sets Ziv Bar-Yossef Lecture 7 April 20,

2 Rank Aggregation

3 Outline The rank aggregation problem Applications Desired properties Arrow’s impossibility theorem Rank aggregation methods

4 The Rank Aggregation Problem m candidates (a.k.a. “alternatives”)  M = {1,…,m}: set of candidates n voters (a.k.a. “agents” or “judges”)  N = {1,…,n}: set of voters Each voter i, has an ranking  i on M   i (a) <  i (b) means i-th voter prefers a to b  Ranking may be a total or partial order The rank aggregation problem: Combine  1,…,  n into a single ranking  on M, which represents the “social choice” of the voters.  Rank aggregation function: f(  1,…,  n ) =    may be a total or partial order

5 Examples m small, n large: elections (multi-party parliament, academies, boards,...) m modest, n small: program committees, sports m large, n small: meta-search, travel plans, restaurant selection

6 Applications to Web Search Meta search  Combine results of different search engines into a better overall ranking Combat spam  Spam results unlikely to rank high in aggregate ranking, even though they can rank high in one or two search engines. Search for multiple terms  AND: bad recall  OR: bad precision  Complex boolean queries: too complicated for average user  Solution: search for small subsets of terms and aggregate results Combine multiple ranking functions  Use different ranking functions (e.g., VSM, PageRank, HITS, …) and aggregate them into a single function

7 Applications to Databases Rank items in a database according to multiple criteria  Ex: Choose a restaurant by cuisine, distance, price, quality, etc.  Ex: Choose a flight ticket by price, # of stops, date and time, frequent flier bonuses, etc.

8 Desired Properties: Unanimity Unanimity (a.k.a. Pareto optimality): If all voters prefer candidate a to candidate b (i.e.,  i (a) <  i (b) for all i), then also  should prefer a to b (i.e.,  (a) <  (b)). aca cab bbc a:b = 3:0

9 Desired Properties: Condorcet Condorcet Criterion [Condorcet, 1785] :  Condorcet winner: a candidate a, which is preferred by most voters to any other candidate b (i.e., for all b, # of i s.t.  i (a) <  i (b) is at least n/2).  Condorcet criterion: If Condorcet winner exists,  should rank it first (i.e.,  (a) = 1). cba aab bcc a:b = 2:1, a:c = 2:1 cba acb bac No Condorcet winner

10 Desired Properties: XCC Extended Condorcet Criterion (XCC):  If most voters prefer candidate a to candidate b (i.e., # of i s.t.  i (a) <  i (b) is at least n/2), then also  should prefer a to b (i.e.,  (a) <  (b)). Not always realizable cba aab bcc  a) <  (b) <  (c) cba acb bac Not realizable

11 XCC and Spam [Dwork et al. 2001] Definition: a page p is said “spam” to a ranking , if there is a page q ranked lower than p, which most human evaluators will think should be ranked higher than p. Assumption: for any two pages p,q, majority of human evaluators agrees with majority of search engine rankings on the order of p,q. Conclusion:  Spam pages are always “Condorcet losers”  If rank aggregation function respects XCC, it eliminates spam.

12 Desired Properties: Independence from Irrelevant Alternatives Independence from Irrelevant Alternatives: Relative order of a and b in  should depend only on relative order of a and b in  1,…,  n.  Ex: if  i = (a b c) changes to (a c b), relative order of a,b in  should not change.

13 Desired Properties: Neutrality and Anonymity Neutrality No candidate should be favored to others.  If two candidates switch positions in  1,…,  n, they should switch positions also in . Anonymity No voter should be favored to others.  If two voters switch their orderings,  should remain the same.

14 Desired Properties: Monotonicity and Consistency Monotonicity If the ranking of a candidate is improved by a voter, its ranking in  can only improve. Consistency If voters are split into two disjoint sets, S and T, and both the aggregation of voters in S and the aggregation of voters in T prefer a to b, then also the aggregation of all voters should prefer a to b.

15 Dictatorship and Democracy Dictatorship: f(  1,…,  n ) =  i Democracy (a.k.a. Majoritian aggregation): Use extended Condorcet Criterion to rank candidates.  Always works for m = 2.  Not always realizable for m ≥ 3.  Theorem [May, 1952]: For m = 2, Democracy is the only rank aggregation function which is monotone, neutral, and anonymous.

16 Arrow’s Impossibility Theorem [Arrow, 1951] Theorem: If m ≥ 3, then the only rank aggregation function that is unanimous and independent from irrelevant alternatives is dictatorship.  Won Nobel prize (1972)

17 Positional Rank Aggregation Methods Plurality  score(a) = # of voters who chose a as #1   : order candidates by decreasing scores Top-k approval  score(a) = # of voters who chose a as one of the top k   : order candidates by decreasing scores Borda’s rule [Borda, 1781]  score(a) =  i  i (a)   : order candidates by increasing scores Violate independence from irrelevant alternatives

18 Positional Methods: Example bcaa dbdb cdcc aabd BordaTop-2 ApprovalPlurality =1022a =931b =1011c =1120d

19 Optimal Rank Aggregation d: distance measure among rankings Definition: The optimal rank aggregation for  1,…,  n w.r.t. d is the ranking  which minimizes  i d( ,  i ).  22 11 nn

20 Distance Measures Kendall tau distance (a.k.a. “bubble sort distance”)  K( ,  ) = # of pairs of candidates (a,b) on which  and  disagree  Ex: K( (a b c d), (a d c b)) = = 3 Spearman footrule distance  F( ,  ) =  a |  (a) -  (a)|  Ex: F((a b c d), (a d c b)) = = 4

21 Kemeny Optimal Aggregation [Kemeny 1959] Optimal aggregation w.r.t. Kendall-tau distance Theorem [Young & Levenglick, 1978] [Truchon 1998] : Kemeny optimal aggregation is the only rank aggregation function, which is neutral, consistent, and satisfies the Extended Condorcet principle.  Effective for fighting spam Generative model:   * is the “correct” ranking   1,…,  n are generated from  by swapping every pair with probability < ½.  Then: Kemeny optimal aggregation gives the maximum likelihood  given  1,…,  n. [Young 1988]

22 Complexity of Kemeny Optimal Aggregation NP-hard, even for n = 4 [Dwork et al. 2001]  In P, for n = 2.  Unknown for n = 3. Can be approximated using Spearman footrule:  Proposition [Diaconis-Graham]: K( ,  ) ≤ F( ,  ) ≤ 2 K( ,  ) What is the complexity of footrule optimal aggregation?

23 Footrule Optimal Aggregation Theorem [Dwork et al. 2001] Footrule optimal aggregation can be computed in polynomial time. Proof  Want to find  which minimizes  i  a |  (a) -  i (a)|  Define a weight bipartite graph G = (L,R,W) as follows: L = M (the candidates) R = {1,…,m}: the available ranks W(a,r) =  i |r -  i (a)|  A matching in G = ranking  Cost of a matching:  i  a |  (a) -  i (a)|  Hence, reduced to finding a minimum cost matching in a bipartite graph

24 Local Kemenization [Dwork et al. 2001] Definition: A ranking  is locally Kemeny optimal aggregation for  1,…,  n if there is no other ranking  ’, which:  Can be obtained from  by flipping one pair  Satisfies  i K(  ’,  i ) <  i K( ,  i ) Features:  Every Kemeny optimal aggregation is also locally Kemeny optimal, but converse is not necessarily true.  Locally Kemeny optimal aggregations satisfy XCC.  Locally Kemeny optimal aggregations can be computed in O(n m log m) time.

25 Markov Chain Techniques [Dwork et al. 2001] Markov Chain states = candidates Transitions depend on the voter rankings Basic idea: probabilistically switch to a “better” candidate Final ranking: induced by stationary distribution

26 Four MC Methods Current state is candidate a. MC1: Choose uniformly from multiset of all candidates that were ranked at least as high as a by some voter.  Probability to stay at a: ~ average rank of a. MC2: Choose a voter i u.a.r. and pick u.a.r. from among the candidates that the i-th voter ranked at least as high as a. MC3: Choose a voter i u.a.r. and pick u.a.r. a candidate b. If i-th voter ranked b higher than a, go to b. Otherwise, stay in a. MC4: Choose a candidate b u.a.r. If most voters ranked b higher than a, go to b. Otherwise, stay in a.  Rank of a ~ # of “pairwise contests” a wins.

27 End of Lecture 7