nips nips2013 nips2013-196 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Prem Gopalan, Chong Wang, David Blei
Abstract: We develop a probabilistic approach for accurate network modeling using node popularities within the framework of the mixed-membership stochastic blockmodel (MMSB). Our model integrates two basic properties of nodes in social networks: homophily and preferential connection to popular nodes. We develop a scalable algorithm for posterior inference, based on a novel nonconjugate variant of stochastic variational inference. We evaluate the link prediction accuracy of our algorithm on nine real-world networks with up to 60,000 nodes, and on simulated networks with degree distributions that follow a power law. We demonstrate that the AMP predicts significantly better than the MMSB. 1
Reference: text
sentIndex sentText sentNum sentScore
1 edu Abstract We develop a probabilistic approach for accurate network modeling using node popularities within the framework of the mixed-membership stochastic blockmodel (MMSB). [sent-6, score-0.627]
2 Our model integrates two basic properties of nodes in social networks: homophily and preferential connection to popular nodes. [sent-7, score-0.295]
3 We develop a scalable algorithm for posterior inference, based on a novel nonconjugate variant of stochastic variational inference. [sent-8, score-0.264]
4 We evaluate the link prediction accuracy of our algorithm on nine real-world networks with up to 60,000 nodes, and on simulated networks with degree distributions that follow a power law. [sent-9, score-0.249]
5 Examples of such networks include online social networks, collaboration networks and hyperlinked blogs. [sent-12, score-0.256]
6 A central problem in social network analysis is to identify hidden community structures and node properties that can best explain the network data and predict connections [19]. [sent-13, score-0.729]
7 Two node properties underlie the most successful models that explain how network connections are generated. [sent-14, score-0.322]
8 The second property that underlies many network models is homophily or similarity, according to which nodes with similar observed or unobserved attributes are more likely to connect to each other. [sent-18, score-0.257]
9 To best explain social network data, a probabilistic model must capture these competing node properties. [sent-19, score-0.384]
10 To capture homophily, our model is built on the mixed-membership stochastic blockmodel (MMSB) [2], a community detection model that allows nodes to belong to multiple communities. [sent-28, score-0.547]
11 (For example, a member of a large social network might belong to overlapping communities of neighbors, co-workers, and school friends. [sent-29, score-0.274]
12 ) The MMSB provides better fits to real network data than single community models [23, 27], but cannot account for node popularities. [sent-30, score-0.584]
13 Specifically, we extend the assortative MMSB [9] to incorporate per-community node popularity. [sent-31, score-0.307]
14 We develop a scalable algorithm for posterior inference, based on a novel nonconjugate variant of stochastic variational inference [11]. [sent-32, score-0.31]
15 Each link denotes a collaboration between two authors, colored by the posterior estimate of its community assignment. [sent-34, score-0.488]
16 Each author node is sized by its estimated posterior popularity and colored by its dominant research community. [sent-35, score-0.442]
17 Following [14], we show an example where incorporating node popularities helps in accurately identifying communities (Right). [sent-37, score-0.557]
18 Further, using simulated networks, we show that node popularities are essential for predictive accuracy in the presence of power-law distributed node degrees. [sent-40, score-0.726]
19 [14] proposed the degree-corrected blockmodel that extends the classic stochastic blockmodels [23] to incorporate node popularities. [sent-44, score-0.394]
20 [16] proposed the latent cluster random effects model that extends the latent space model [10] to include node popularities. [sent-46, score-0.311]
21 Both models capture node similarity and popularity, but assume that unobserved similarity arises from each node participating in a single community. [sent-47, score-0.57]
22 Finally, the Poisson community model [4] is a probabilistic model of overlapping communities that implicitly captures degree-corrected mixedmemberships. [sent-48, score-0.457]
23 However, the standard EM inference under this model drives many of the per-node community parameters to zero, which makes it ineffective for prediction or model metrics based on prediction (e. [sent-49, score-0.329]
24 2 Modeling node popularity and similarity The assortative mixed-membership stochastic blockmodel (MMSB) [9] treats the links or non-links yab of a network as arising from interactions between nodes a and b. [sent-52, score-1.25]
25 Each node a is associated with community memberships πa , a distribution over communities. [sent-53, score-0.702]
26 The probability that two nodes are linked is governed by the similarity of their community memberships and the strength of their shared communities. [sent-54, score-0.707]
27 Given the communities of a pair of nodes, the link indicators yab are independent. [sent-55, score-0.533]
28 We draw yab repeatedly by choosing a community assignment (za→b , za←b ) for a pair of nodes (a, b), and drawing a binary value from a community distribution. [sent-56, score-1.032]
29 Specifically, the conditional probability of a link in MMSB is p(yab = 1|za→b,i , za←b,j , β) = K i=1 K j=1 za→b,i za←b,j βij , where β is the blockmodel matrix of community strength parameters to be estimated. [sent-57, score-0.492]
30 This captures node similarity in community memberships—if two nodes are linked, it is likely that the latent community indicators were the same. [sent-59, score-1.081]
31 2 In the proposed model, assortative MMSB with node popularities, or AMP, we introduce latent variables θa to capture the popularity of each node a, i. [sent-60, score-0.739]
32 , its propensity to attract links independent of its community memberships. [sent-62, score-0.408]
33 We capture the effect of node popularity and community similarity on link generation using a logit model logit (p(yab = 1|za→b , za←b , θ, β)) ≡ θa + θb + where we define indicators same community k. [sent-63, score-1.193]
34 This model is also similar in the spirit of the random effects model [10]—the node-specific K k effect θa captures the popularity of individual nodes while the k=1 δab βk term captures the interactions through latent communities. [sent-72, score-0.406]
35 For each node a, (a) Draw community memberships πa ∼ Dirichlet(α). [sent-78, score-0.702]
36 (c) Draw the probability of a link yab |za→b , za←b , θ, β ∼ logit−1 (za→b , za←b , θ, β). [sent-83, score-0.355]
37 Under the AMP, the similarities between the nodes’ community memberships and their respective popularities compete to explain the observations. [sent-84, score-0.686]
38 We can make AMP simpler by replacing the vector of K latent community strengths β with a single community strength β. [sent-85, score-0.711]
39 We analyze data with the AMP via the posterior distribution over the latent variables 2 2 p(π1:N , θ1:N , z, β1:K |y, α, µ0 , σ0 , σ1 ), where θ1:N represents the node popularities, and the posterior over π1:N represents the community memberships of the nodes. [sent-87, score-0.83]
40 This results in posterior estimates of the community memberships and popularities for each node and posterior estimates of the community assignments for each link. [sent-92, score-1.279]
41 With these estimates, we visualized the discovered community structure and the popular authors. [sent-93, score-0.326]
42 In general, with an estimate of this latent structure, we can study individual links, characterizing the extent to which they occur due to similarity between nodes and the extent to which they are an artifact of the popularity of the nodes. [sent-94, score-0.379]
43 3 A stochastic gradient algorithm for nonconjugate variational inference 2 2 Our goal is to compute the posterior distribution p(π1:N , θ1:N , z, β1:K |y, α, µ0 σ0 , σ1 ). [sent-95, score-0.353]
44 First, in variational inference the coordinate updates are available in closed form only when all the nodes in the graphical model satisfy conditional conjugacy. [sent-99, score-0.363]
45 To see this, note that the Gaussian priors on the popularity θ and the community strengths β are not conjugate to the conditional likelihood of the data. [sent-101, score-0.491]
46 Second, coordinate ascent algorithms iterate over all the O(N 2 ) node pairs making inference intractable for large networks. [sent-102, score-0.381]
47 We use the mean-field family, with the following variational distributions: q(za→b = i, za←b = j) = φij ; ab q(βk ) = 2 N (βk ; µk , σβ ); q(πn ) = Dirichlet(πn ; γn ); 2 q(θn ) = N (θn ; λn , σθ ). [sent-107, score-0.288]
48 (2) The posterior over the joint distribution of link community assignments per node pair (a, b) is parameterized by the per-interaction memberships φab 1 , the community memberships by γ, the community strength distributions by µ and the popularity distributions by λ. [sent-108, score-1.832]
49 The remaining lines contain summations over all node pairs; we call these local terms. [sent-115, score-0.303]
50 The distinction between the global and local parameters is important—the updates to global parameters depends on all (or many) local parameters, while the updates to local parameters for a pair of nodes only depends on the relevant global and local parameters in that context. [sent-117, score-0.508]
51 Coordinate ascent inference must consider each pair of nodes at each iteration, but even a single pass through the O(N 2 ) node pairs can be prohibitive. [sent-119, score-0.529]
52 Nevertheless, by carefully manipulating the variational objective, we can develop a scalable stochastic variational inference algorithm for the AMP. [sent-122, score-0.354]
53 The data likelihood terms in the ELBO can be written as Eq [log p(yab |za→b , za←b , θ, β)] = yab Eq [xab ] − Eq [log(1 + exp(xab ))], (4) K k where we define xab ≡ θa + θb + k=1 βk δab . [sent-125, score-0.357]
54 6: global step 7: Update memberships γa , for each node a ∈ S, using stochastic natural gradients in Eq. [sent-137, score-0.585]
55 8: Update popularities λa , for each node a ∈ S using stochastic gradients in Eq. [sent-139, score-0.553]
56 9: Update community strengths µ using stochastic gradients in Eq. [sent-141, score-0.446]
57 10: Set ρa (t) = (τ0 + ta )−κ ; ta ← ta + 1, for each node a ∈ S. [sent-143, score-0.332]
58 3 The global step We optimize the ELBO with respect to the global variational parameters using stochastic gradient ascent. [sent-157, score-0.337]
59 The global step updates the global community memberships γ, the global popularity parameters λ and the global community strength parameters µ with a stochastic gradient of the lower bound on the ELBO L . [sent-162, score-1.315]
60 In [9], the authors update community memberships of all nodes after each iteration by obtaining the natural gradients of the ELBO 2 with respect to the vector γ of dimension N × K. [sent-163, score-0.651]
61 We use natural gradients for the memberships too, but use distinct stochastic optimizations for the memberships and popularity parameters of each node and maintain a separate learning rate for each node. [sent-164, score-0.868]
62 Since the variational objective is a sum of terms, we can cheaply compute a stochastic gradient by first subsampling a subset of terms and then forming an appropriately scaled gradient. [sent-166, score-0.261]
63 At each iteration we sample a node uniformly at random from the N nodes in the network. [sent-168, score-0.379]
64 ) While a naive method will include all interactions of a sampled node as the observed pairs, we can leverage network sparsity for efficiency; in many real networks, only a small fraction of the node pairs are linked. [sent-170, score-0.601]
65 Following [2, 9], we have t−1 t ∂γa,k = −γa,k + αk + (a,b)∈links(a) φkk (t) + ab (a,b)∈nonlinks(a) φkk (t), ab (6) where links(a) and nonlinks(a) correspond to the set of links and non-links of a in the training set. [sent-173, score-0.457]
66 Therefore, the gradient of L with respect to the membership parameter γa , computed using all of the nodes’ links and a subsample of its non-links, is a noisy but unbiased estimate of the natural gradient in Eq. [sent-176, score-0.288]
67 5 The gradient of the approximate ELBO with respect to the popularity parameter λa is ∂λt = − a t−1 λa 2 σ1 + (a,b)∈links(a) ∪ nonlinks(a) (yab − rab sab ), (7) where we define rab as rab ≡ 2 2 exp{λa +σθ /2} exp{λb +σθ /2} . [sent-180, score-0.698]
68 2 2 1+exp{λa +σθ /2} exp{λb +σθ /2}sab (8) Finally, the stochastic gradient of L with respect to the global community strength parameter µk is ∂µt = k t−1 µ0 −µk 2 σ0 + N 2|S| (a,b)∈links(S) ∪ nonlinks(S) 2 φkk (yab − rab exp{µk + σβ /2}). [sent-181, score-0.642]
69 ab (9) As with the community membership gradients, notice that an unbiased estimate of the summation term over non-links in Eq. [sent-182, score-0.472]
70 To obtain an unbiased estimate of the true gradient with respect to µk , the summation over a node’s links and non-links must be scaled by the inverse probability of subsampling that node in Eq. [sent-185, score-0.439]
71 Since each pair is shared between two nodes, and we use a mini-batch with S nodes, the summations over N the node pairs are scaled by 2|S| in Eq. [sent-187, score-0.343]
72 7, (yab − rab sab ) is the residual for the pair (a, b), while in Eq. [sent-195, score-0.25]
73 9, (yab − rab exp{µk + 2 σβ /2}) is the residual for the pair (a, b) conditional on the latent community assignment for both nodes a and b being set to k. [sent-196, score-0.631]
74 Further, notice that the updates for the global parameters of node a and b, and the updates for µ depend only on the diagonal entries of the indicator variational matrix φab . [sent-197, score-0.477]
75 (10) We maintain separate learning rates ρa for each node a, and only update the γ and λ for the nodes in the mini-batch in each iteration. [sent-201, score-0.379]
76 There is a global learning rate ρ for the community strength parameters µ, which are updated in every iteration. [sent-202, score-0.395]
77 There is a per-interaction variational parameter of dimension K × K for each node pair—φab —representing the posterior approximation of which pair of communities are active in determining the link or non-link. [sent-209, score-0.646]
78 The coordinate ascent update for φab is 2 φkk ∝ exp Eq [log πa,k ] + Eq [log πb,k ] + yab µk − rab (exp{µk + σβ /2} − 1) ab (11) φij ab (12) ∝ exp Eq [log πa,i ] + Eq [log πb,j ] , i = j, where rab is defined in Eq. [sent-210, score-1.014]
79 5 Initialization and convergence We initialize the community memberships γ using approximate posterior memberships from the variational inference algorithm for the MMSB [9]. [sent-214, score-0.857]
80 We initialized popularities λ to the logarithm of the normalized node degrees added to a small random offset, and initialized the strengths µ to zero. [sent-215, score-0.492]
81 N is the number of nodes, d is the percent of node pairs that are links and P is the mean perplexity over the links and nonlinks in the held-out test set. [sent-220, score-0.642]
82 4 Empirical study We use the predictive approach to evaluating model fitness [8], comparing the predictive accuracy of AMP (Algorithm 1) to the stochastic variational inference algorithm for the MMSB with link sampling [9]. [sent-285, score-0.415]
83 We computed the link prediction accuracy using a test set of node pairs that are not observed during training. [sent-294, score-0.366]
84 We approximate the predictive distribution of a held-out node pair yab under the AMP using ˆ ˆ posterior estimates θ, β and π as ˆ p(yab |y) ≈ 3 za→b za←b ˆ ˆ p(yab |za→b , za←b , θ, β)p(za→b |ˆa )p(za←b |ˆb ). [sent-297, score-0.627]
85 020 2 4 6 8 2 4 6 8 amp mmsb 2 4 6 8 Ratio of max. [sent-319, score-0.696]
86 Perplexity is the exponential of the average predictive log likelihood of the held-out node pairs. [sent-325, score-0.312]
87 For mean precision and recall, we generate the top n pairs for each node ranked by the probability of a link between them. [sent-326, score-0.413]
88 The ranked list of pairs for each node includes nodes in the test set, as well as nodes in the training set that were non-links. [sent-327, score-0.555]
89 For the stochastic AMP algorithm, we set the “mini-batch” size S = N/100, where N is the number of nodes in the network and we set the non-link sample size m0 = 100. [sent-332, score-0.266]
90 We set the number of communities K = 2 for the political blog network and K = 20 2 for the US air; for all other networks, K was set to 100. [sent-333, score-0.282]
91 The first four networks are small in size, and were fit using the AMP model with a single community strength parameter. [sent-345, score-0.404]
92 All other networks were fit with the AMP model with K community strength parameters. [sent-346, score-0.404]
93 Without node popularities, MMSB is dependent entirely on node memberships and community strengths to predict links. [sent-348, score-0.992]
94 Since K is held fixed, communities are likely to have more nodes as N increases, making it increasingly difficult for the MMSB to predict links. [sent-349, score-0.256]
95 For the AMP the mean precision at 10 for US Air, political blogs and netscience were 0. [sent-351, score-0.256]
96 We set a community size range of [200, 500] and a mean node degree of 10 with power-law exponent set to 2. [sent-361, score-0.554]
97 The skewness in degree distributions causes the community strength parameters of MMSB to overestimate or underestimate the linking patterns within communities. [sent-364, score-0.398]
98 The per-node popularities in the AMP can capture the heterogeneity in node degrees, while learning the corrected community strengths. [sent-365, score-0.724]
99 Benchmarks for testing community detection algorithms on directed and weighted graphs with overlapping communities. [sent-510, score-0.317]
100 Finding community structure in networks using the eigenvectors of matrices. [sent-545, score-0.346]
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