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Community Detection with Negative Links

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Presentation at ETH, June 9, 2009.

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Community Detection with Negative Links

  1. 1. Community Detection with Negative Links Vincent Traag1 Jeroen Bruggeman2 1Catholic University of Louvain, Belgium 2University of Amsterdam, Netherlands June 9, 2009 Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 1 / 15
  2. 2. Outline 1 Introduction 2 Social Balance Theory 3 Modularity 4 Including negative links 5 Empirical example Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 2 / 15
  3. 3. Introduction • Community detection is succesfully applied in a number of fields. • Whether a link is positive or negative usually ignored. • It is highly relevant for • Hyperlinks on webpages (“good” sites, instead of “important” sites) • References in blogs (opinion clustering, not thematical) • Trust relationships (e.g. P2P systems) • International relationships (conflict and cooperation) • . . . Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 3 / 15
  4. 4. Social Balance Theory C1 C2 AB C D • Triads (sets of three nodes) are balanced if their relationships are “symmetric”. • Triad i, j, k is balanced if AijAikAjk = 1. • If network is balanced, is can be split in two clusters. (Harary, 1953) • A network is said to be k-balanced if it can be split into k clusters. • For unbalanced networks, how can the nodes be clustered? Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 4 / 15
  5. 5. Social Balance Theory C1 C2 C3 AB D E C • Triads (sets of three nodes) are balanced if their relationships are “symmetric”. • Triad i, j, k is balanced if AijAikAjk = 1. • If network is balanced, is can be split in two clusters. (Harary, 1953) • A network is said to be k-balanced if it can be split into k clusters. • For unbalanced networks, how can the nodes be clustered? Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 4 / 15
  6. 6. Frustration • Try to come close to the ideal ’balanced’ network. • Minimize links that violate the conditions of k-balance: • Negative links within clusters, • Positive links between clusters. Definition Frustration F = ij αA− ij δ(σi , σj ) + (1 − α)A+ ij (1 − δ(σi , σj )). • If α = 1 2 this is equivalent to minimizing F = ij (A+ ij − A− ij )δ(σi , σj ) = ij Aij δ(σi , σj ). Approach by Doreian and Mrvar, Social Networks, Vol. 18, (1996). Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 5 / 15
  7. 7. Problems with frustration • If there are no negative links, there is only one cluster. • Even minimally postive connected group is in one cluster. • Absent links do not join or seperate a cluster. • Defines unclearly a community. Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 6 / 15
  8. 8. Modularity Modularity has been succesfully applied in community detection. Definition (Modularity) Q = 1 m ij (Aij − pij )δ(σi , σj ) = 1 m c ac − ec. Newman & Girvan, Phys Rev E 69, (2004). Maximizing modularity yields a ”good” community assignment. Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 7 / 15
  9. 9. Potts approach • Potts approach by Reichardt and Bornholdt (2006): reward “allowed” links, penalise “forbidden” links. Allowed • Links within communities (reward aij = 1 − γpij). Forbidden • Absent links within communities (penalty bij = γpij). • Formulated as an “energy/cost” function (Hamiltonian): H = ij −aijAijδ(σi , σj ) + bij (1 − Aij)δ(σi , σj ) • Reformulated equals modularity (if γ = 1) Q = − 1 m H = 1 m ij (Aij − γpij )δ(σi , σj ) • Results in a tuneable (γ) version of modularity. Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 8 / 15
  10. 10. Problem with negative links ak = 1 b k = 1 c k = −1 Negative links poses problem for modularity. Expected values pij not well defined. A =   + + − + + − − − +   Q = 1 m ij Aij − ki kj m δ(σi , σj ) = 0 Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 9 / 15
  11. 11. Allowing negative links • Solution is to separate the positive and negative part. • Then change “allowed” and “forbidden” links: Allowed • Positive links within communities (reward aij = γp+ ij ). • Absent negative links within communities (reward dij = λp− ij ). Forbidden • Absent positive links within communities (penalty bij = 1 − γp+ ij ). • Negative links within communities (penalty cij = 1 − λp− ij ). • Results in two separate Hamiltonians H+ = − ij (A+ ij − γp+ ij )δ(σi , σj ) and H− = ij (A− ij − λp− ij )δ(σi , σj ). Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 10 / 15
  12. 12. Hamiltonian • When both Hamiltonians are weighted equally this equals minimizing H = H+ + H− = ij (Aij − (γp+ ij − λp− ij ))δ(σi , σj ) • This is similar to modularity, but with different expected values. • If there are no negative links, (and γ = 1) this equals modularity. • Equivalent to choosing the appropriate null-model. • If γ = λ = 0, or if graph is complete and balanced this is equal to minimizing frustration. • Implemented in the simulated annealing scheme used by Reichardt & Bornholdt (2006). Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 11 / 15
  13. 13. Empirical example γ = 1, λ = 1 Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 12 / 15
  14. 14. Empirical example γ = 0.3, λ = 1 Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 13 / 15
  15. 15. Empirical example γ = 1, λ = 2 Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 14 / 15
  16. 16. Conclusions • Proposed a solution for finding communities with negative links. • Is in agreement with techniques for community detection with positive links only. • Results similar for ”social balance” clustering if network is (almost complete) and balanced. • Yields good community assignments. • Can be readily implemented in existing modularity optimization techniques. Vincent Traag (UC Louvain) Community Detection with Negative Links June 9, 2009 15 / 15

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