Outlier Pursuit: Robust PCA and Collaborative Filtering

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1 Outlier Pursuit: Robust PCA and Collaborative Filtering Huan Xu Dept. of Mechanical Engineering & Dept. of Mathematics National University of Singapore Joint w/ Constantine Caramanis, Yudong Chen, Sujay Sanghavi

2 Outline PCA and Outliers - Why SVD fails - Corrupted features vs. corrupted points Our idea + Algorithms Results - Full observation - Missing Data Framework for Robustness in High Dimensions

3 Principal Components Analysis Given points that lie on/near a Lower dimensional subspace, find this subspace. Data Points Classical technique: 1. Organize points as matrix 2. Take SVD 3. Top singular vectors span space Low Rank Matrix

4 Fragility Gross errors of even one/few points can completely throw off PCA Reason: Classical PCA minimizes error, which is susceptible to gross outliers

5 Two types of gross errors Corrupted Features Outliers - Individual entries corrupted -Entire columns corrupted.. and missing data versions of both

6 PCA with Outliers Points Objective: find identities of outliers (and hence col. space of true matrix)

7 Outlier Pursuit - Idea Points Standard PCA

8 Outlier Pursuit - Method Points We propose: Convex surrogate for Rank constraint Convex surrogate for Column-sparsity

9 When does it (not) work? When certain directions of column space of poorly represented This vector has large inner product with some coordinate axes is large

10 Results Assumption: Columns of true are incoherent: Note: First consider: Noiseless case

11 Results Assumption: Columns of true are incoherent: Note: Theorem: (noiseless case) Our convex program can identify upto a fraction of outliers as long as Outer bound: makes the problem un-identifiable

12 Proof Technique A point is the optimum of a convex function Steps: 1. guess a nice point, -- oracle problem Zero lies in the (sub) gradient of at 2. show it is the optimum by showing zero is in subgradient

13 Proof Technique Guessing a nice optimum (Note: Due to the fact that column sparse matrix is also low-rank, hence nice points are non-unique. In problems like matrix completion, compressed sensing or RPCA with sparse corruption, this is not an issue) Oracle Problem: is, by definition, a nice point. Rest of proof: showing it is the optimum of original program, under our assumption.

14 Proof Technique Constructing Dual Certificate: need a Q to satisfy A first guess is to use This works when each corrupted column is orthogonal, but fails otherwise. To satisfy the equalities, use a corrected Q. Then show the inequalities hold under the conditions given in the theorem.

15 (More detailed) Proof Technique One can verify that Let then (1) and (2) are satisfied To satisfy (3), we need, such that This can be constructed via Indeed the least square solution. The inverse operator =, provided the series converges. satisfies all three equalities. Then show the inequalities hold.

16 Noisy case We observe for unknown noise Relax the equality constraint to inequality, i.e., solve Under essentially same condition as the noiseless case, the noisy outlier pursuit succeeds: it finds a solution close to a pair with correct column space and column support.

17 Implementation Issue Outlier Pursuit is an SDP. Interior methods may not scale. We are interested in the structure can sacrifice accuracy for scalability Use proximal gradient algorithms.

18 Performance L + C formulation L + S formulation ( from [Chandrasekaran et. al.], [Candes,et. al.] )

19 More experiment 220 samples of digit 1 and 11 samples of digit 7, unlabeled.

20 Another view Mean is solution of Standard PCA of M is solution of Fragile: Can be easily skewed by one / few points Median is solution of Our method is (convex rel. of) Robust: skewing requires Error in constant fraction of pts

21 Collaborative Filtering w/ Adversaries

22 In an incident that highlights the pitfalls of online recommendation systems, Amazon.com on Friday removed a link to a sex manual that appeared next to a listing for a spiritual guide by well-known Christian televangelist Pat Robertson.

23 Manipulation in RS Recommendation systems utilize ALL users ratings to predict theirs future preferences. Malicious users try to skew prediction by injecting fake ratings. Some popular algorithms (e.g. knn) are vulnerable to manipulation.

24 RS via Matrix Completion (convention wisdom) Preference Matrix Partial Observations Given Recover L 0?

25 RS via Matrix Completion Assumption: User Preferences form Low-rank Matrix Users Products Features User Features Products User 3 s overall preference for Product 2 Product 2 s feature vector User 3 s feature vector e.g. Product = Laptop Performance Portability Preference for performance Preference for portability

26 Robust Matrix Completion (New idea) M L 0 (Low-rank) Authentic columns C 0 (Column-Sparse) Arbitrarily corrupted columns Partial Observation Problem: Given, find L 0 and indentify columns of C 0

27 Collaborative Filtering w/ Adversaries Users Low-rank matrix that -Is partiallly observed -Has some corrupted columns == outliers with missing data! Our setting: -Good users == random sampling of incoherent matrix (as in matrix completion) -Manipulators == completely arbitrary sampling, values

28 Outlier Pursuit with Missing Data for observed Now: need row space to be incoherent as well - since we are doing matrix completion and manipulator identification

29 Our Result Theorem: Convex program optimum is such that has the correct column space and the support of is exactly the set of manipulators, whp, provided Sampling density Fraction of users that are manipulators Note: no assumptions on manipulators

30 Corollaries The algorithm succeeds if there are a constant fraction of corrupted columns, and the fraction of observation is also a constant; there are a growing number of corrupted columns, and the fraction of observations goes to zero.

31 Roadmap of the Proof Need a Q to satisfy Least square solution to satisfy (1), (2), and (5)? Problem: least square solution involves infinite series, which breaks down independence, while to get sharp inequality, we need to exploit iid of sampling.

32 Roadmap of the proof -2 Rethink of the optimality condition: Optim. certificate insures for any feasible pertubation The condition, comes from the need to show above inequality for any However, that is not necessary. One can show that We can relax the equality condition to inequality, but tighten the other inequalities. Makes it possible to avoid infinite series.

33 Robust Collaborative Filtering Algo: Partially observed Low-rank + Column-sparse Algo: Partially observed Sparse + Low-rank

34 More generally Several methods in High-dim. Statistics Loss function regularizer Our approach: (same) Loss function Weighted sum of regularizers Yields robustness + flexibility in several settings. Today: PCA wit Outliers + missing data

35 Papers can be found in my website:

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