# Part 2: Community Detection

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1 Chapter 8: Graph Data Part 2: Community Detection Based on Leskovec, Rajaraman, Ullman 2014: Mining of Massive Datasets Big Data Management and Analytics

2 Outline Community Detection - Social networks - Betweenness - Girvan-Newman Algorithm - Modularity - Graph Partitioning - Spectral Graph Partitioning - Trawling Big Data Management and Analytics 2

3 Networks & Communities: Think of networks being organized into: Modules Cluster Communities Goal: Find densely linked clusters Big Data Management and Analytics 3

4 What is a Social Network? Characteristics of a social network: - Collection of entities participating in the network (entities might be individuals, phone numbers, addresses, ) - At least one relationship between entities of the network. (Facebook: friend ). Relationship can be all-or-nothing or specified by a degree (e.g. fraction of the average day that two people communication to each other) - Assumption of nonrandomness or locality, i.e. relationships tend to cluster. (e.g. A is related to B and C higher probability that B is related to C) Big Data Management and Analytics 4

5 How to find communities? Here we will work with undirected (unweighted networks) We need to resolve 2 questions: How to compute betweenness? How to select the number of clusters? Big Data Management and Analytics 5

6 Outline Community Detection - Social networks - Betweenness - Girvan-Newman Algorithm - Modularity - Graph Partitioning - Spectral Graph Partitioning - Trawling Big Data Management and Analytics 6

7 Betweenness Definition: The betweenness of an edge (a,b) is the number of pairs of nodes x and y such that (a,b) lies on the shortest-path between x and y. Example: Edge (B,D) has highest betweenness (shortest path of A,B,C to any of D,E,F,G) Betweenness of (B,D) aggregates to: 3 x 4 = 12 Betweenness of edge (D,F)? Big Data Management and Analytics 7

8 Girvan-Newman Algorithm Goal: Computation of betweenness of edges Step 1: Perform a breadth-first search, starting at node X and construct a DAG (directed, acyclic graph) Example: - Start at node E Level 1 Level 2 Level 3 Big Data Management and Analytics 8

9 Girvan-Newman Algorithm Goal: Computation of betweenness of edges Step 2: label each node by the number of shortest paths that reach it from the root. Label of root = 1, each node is labeled by the sum of its parents. Example: Level 1 Level Level 3 Big Data Management and Analytics 9

10 Girvan-Newman Algorithm Goal: Computation of betweenness of edges Step 3: calculate for each edge e the sum over all nodes Y the fraction of shortest paths from the root X to Y: In Detail: 1. Each leaf gets a credit of Non-leaf nodes gets a credit of 1 plus the sum of its children 3. A DAG edge e entering node Z from the level above is given a share of the credit of Z proportional to the fraction of shortest paths from the root to Z. Formally: let Y 1,, Y k be the parent nodes of Z with p i, 1 i k, be the number of shortest path to Y i. The credit k for edge (Y i, Z) is given by: Z p i / j=1 p j Level 1 Level 2 Level 3 Big Data Management and Analytics 10

11 Find Communities using Betweenness Idea: Clustering is performed by taking the edges in order of increasing betweenness and add them to the graph or as a process of edge removal Example: GN-Algorithm has been performed for every node and the credit of each edge has been calculated (by summing the credits up and dividing them by 2. Why?) Remove edges, starting by highest betweenness: - 1. Remove (B,D) Communities {A,B,C} and {D,E,F,G} - 2. Remove (A,B), (B,C), (D,G), (D,E), (D,F) Communities {A,C} and {E,F,G} Node B and D are encapsulated as traitors of communities Big Data Management and Analytics

12 Find Communities using Betweenness Girvan-Newman Algorithm: - connected components are communities - gives a hierarchical decomposition of the network Big Data Management and Analytics 12

13 Outline Community Detection - Social networks - Betweenness - Girvan-Newman Algorithm - Modularity - Graph Partitioning - Spectral Graph Partitioning - Trawling Big Data Management and Analytics 13

14 Network Communities How to compute betweenness? How to select the number of clusters? Communities: sets of tightly connected nodes Modularity Q: - A measure of how well a network is partitioned into communities - Given a partitioning of the network into groups s S: Q s S [ #edges within group s (expected #edges within group s)] Defined by null model Big Data Management and Analytics 14

15 Null Model: Configuration Model - Given a graph G with n nodes and m edges, construct rewired network G : - Same degree distribution but random connections - Consider G as a multigraph The expected number of edges between nodes i and j of degrees k i and k j is given by: 1 2m k ik j Proof that G contains the expected number of m edges: 1 k i k j 2 2m = 1 1 k 2 2m i i N j N i N j N k j = 1 2m 2m = m 4m Big Data Management and Analytics 15

16 Modularity Modularity of partitioning S of graph G: Q s S [ #edges within group s (expected #edges within group s)] Q G, S = 1 2m s S i s j s (a ij k ik j 2m ) Normalizing: 1 < Q < 1 Modularity values take range [ 1, 1]: - Positive if the number of edges within groups exceeds the expected number < Q means significant community structure Big Data Management and Analytics 16

17 Modularity Q is useful for selecting the number of clusters Big Data Management and Analytics 17

18 Outline Community Detection - Social networks - Betweenness - Girvan-Newman Algorithm - Modularity - Graph Partitioning - Spectral Graph Partitioning - Trawling Big Data Management and Analytics 18

19 Partitioning of Graphs Given an undirected Graph G(V, E): Bi-partitioning task: - Divide vertices into two disjoint groups A, B A B Questions: - How can we define good partition of G? - How can we efficiently identify such a partition? Big Data Management and Analytics 19

20 Partitioning of Graphs What makes a good partition? Maximize the number of within-group connections Minimize the number of between-group connections Example: A B Big Data Management and Analytics 20

21 Partitioning of Graphs Graph Cuts Express partitioning objectives as a function of the edge cut of the partition Cut: Set of edges with only one vertex in a group: Example: A cut A, B = i A,j B w ij B cut A, B = 2 Big Data Management and Analytics 21

22 Partitioning of Graphs Minimum-cut Minimize weight of connections between groups: arg min cut(a, B) A,B Example: Optimal Cut Minimum Cut Problem: Only considers external cluster connections Does not consider internal cluster connectivity Big Data Management and Analytics 22

23 Partitioning of Graphs - Graph Cuts Normalized-cut: Connectivity between groups relative to the density of each group cut(a, B) cut(a, B) ncut A, B = + vol(a) vol(b) vol(x): total weight of edges with at least one endpoint in X: vol X = i A k i Produces more balanced partitions How to find a good partition efficiently? Problem: Computing optimal cut is NP-hard! Big Data Management and Analytics 23

24 Spectral Graph Partitioning - Adjacency matrix of an undirected Graph G a ij = 1 if (i, j) exist in G, else 0 - Vector x R n with components x 1,, x n Think of it as a label/value of each node of G What is the meaning of A x? a 11 a 1n a n1 a nn x 1 x n = y 1 y n y i = n j=1 a ij x j = (i,j) E x j Entry y i is a sum of labels / values x j of neighbors of i Big Data Management and Analytics 24

25 Spectral Graph Partitioning What is the meaning of A x? a 11 a 1n a n1 a nn x 1 x n = y 1 y n a 11 a 1n a n1 a nn x 1 x n = λ x 1 x n A x = λ x Eigenvalue Problem Spectral Graph Theory: Analyze the spectrum of matrix representing G Spectrum: Eigenvectors x i of a graph, ordered by the magnitude (strength) of their corresponding eigenvalues λ i Λ = λ 1,, λ n with λ 1 λ n Big Data Management and Analytics 25

26 Spectral Graph Partitioning Intuition Suppose all nodes in G have degree d and G is connected What are some eigenvalues/vectors of G? Eigenvalue Problem: A x = λ x find λ and x Let s try x = (1,, 1) Then A x = d,, d = λ x λ = d Remember: y i = a ij x j = j=1 (i,j) E Big Data Management and Analytics 26 n x j

27 Spectral Graph Partitioning Intuition What if G is not connected? A G has 2 components, each d-regular What are some eigenvectors? x = put all 1s on A and 0s on B or vice versa x = (1,, 1, 0,, 0), then A x = (d,, d, 0,, 0) x = (0,, 0, 1,, 1), then A x = (0,, 0, d,, d) in both cases the corresponding λ = d B A B A B λ n = λ n 1 λ n λ n nd largest eigval. λ n 1 now has value very close to λ n Big Data Management and Analytics 27

28 Spectral Graph Partitioning Intuition A B A B λ n = λ n 1 λ n λ n nd largest eigval. λ n 1 now has value very close to λ n If the graph is connected (right example) then we already know that x n = (1,, 1) is an eigenvector Since eigenvectors are orthogonal then the components of x n 1 sum to 0 Why? Because i x n [i] i x n 1 i = 0 General Idea: we can look at the eigenvector of the 2nd largest eigenvalue and declare nodes with positive label in A and negative label in B Big Data Management and Analytics 28

29 Spectral Graph Partitioning - Adjacency matrix A: n x n matrix A = a ij, a ij = 1 if there is an edge between node i and j Big Data Management and Analytics 29

30 Spectral Graph Partitioning - Degree matrix D: n x n diagonal matrix D = d ii, d ii = degree of node i Big Data Management and Analytics 30

31 Spectral Graph Partitioning - Laplacian Matrix L: n x n symmetric matrix L = D A Trivial eigenpair? X = (1,, 1), then L x = 0 and so λ 1 = Big Data Management and Analytics 31

32 Spectral Clustering Algorithms Three basic stages: 1. Pre-processing Construct a matrix representation of the graph 2. Decomposition Compute eigenvalues and eigenvectors of the matrix Map each point point to a lower-dimensional representation on one or more eigenvectors 3. Grouping Assign points to two or more clusters, based on the new representation Big Data Management and Analytics 32

33 Spectral Clustering Algorithms 1. Pre-processing: Build Laplacian matrix L of the graph Decomposition: Find eigenvalues λ and eigenvectors x of the matrix L = X = Map vertices to lower-dimensional representation How do we now find the clusters? Big Data Management and Analytics 33

34 Spectral Clustering Algorithms 3. Grouping: Sort components of reduced 1-dimensional vector Identify clusters by splitting the sorted vector in two (threshold ε) By choosing m vectors, there are max. 2 m clusters How to choose a splitting point, i.e threshold ε? Naive approaches: Split at ε = 0 or median value More expensive approaches: Attempt to minimize normalized cut in 1-dimension (sweep over ordering of nodes induces by the eigenvector) Split at ε = 0: Cluster A: Positive points Cluster B: Negative points A B Big Data Management and Analytics

35 Spectral Clustering Algorithms Value of x 2 Rank in x 2 Big Data Management and Analytics

36 Spectral Clustering Algorithms Components of x 2 Value of x 2 Rank in x 2 Big Data Management and Analytics

37 Spectral Clustering Algorithms Components of x 3 Big Data Management and Analytics

38 Outline Community Detection - Social networks - Betweenness - Girvan-Newman Algorithm - Modularity - Graph Partitioning - Spectral Graph Partitioning - Trawling Big Data Management and Analytics 38

39 - Trawling Trawling Goal: find small communities in huge graphs E.g. how to describe community/discussion in a Web Example: E.g. people talking about the same things or visited web pages Big Data Management and Analytics 39

40 - Trawling Problem definition: Enumerate complete bipartite subgraphs K s,t : All vertices in K s,t can be partitioned in two sets. Each vertex in the first set of size s is linked to each vertex in second set of size t Where K s,t : s nodes on the left where each links to the same t other nodes on the right X = s = 3 Y = t = 4 X Y Big Data Management and Analytics 40

41 - Trawling Frequent Itemset Analysis Market Basket Analysis Market: Universe U of n items Baskets: subsets of U: S 1, S 2,, S m U (S i is a set of items one person bought) Support: frequency threshold Goal: Find all subsets T s.t. T S i of at least f sets S i (items in T were bought together at least f times) Frequent itemsets = complete bipartite graphs i a b c d S i ={a,b,c,d} View each node i as a set S i of nodes i points to K s,t = a set Y of size t that occurs in s sets S i j i k a b c d Big Data Management and Analytics 41

42 - Trawling E.g. Bipartite subgraph K 3,4 a frequent itemset Y={a,b,c} of supp s. So, there are s nodes that link to all of {a,b,c}: x a b c y a b c z a b c x y z a b c We found K s,t! K s,t = a set Y of size t that occurs in s sets S i Big Data Management and Analytics 42

43 - Trawling a c e f d b Itemsets: a = {b,c,d} b = {d} c = {b,d,e,f} d = {e,f} e = {b,d} f = {} Frequent itemsets support > 1 {b,d}: support 3 {e,f}: support 2 a c d b c e f d e Big Data Management and Analytics 43

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