Paper:
Mixture Matrix Completion
Completing a data matrix X has become an ubiquitous problem in modern data science, with motivations in recommender systems, computer vision, and networks inference, to name a few. One typical assumption is that X is low-rank. A more general model assumes that each column of X corresponds to one of several lowrank matrices. This paper generalizes these models to what we call mixture matrix completion (MMC): the case where each entry of X corresponds to one of several low-rank matrices. MMC is a more accurate model for recommender systems, and brings more flexibility to other completion and clustering problems. We make four fundamental contributions about this new model. First, we show that MMC is theoretically possible (well-posed). Second, we give its precise information-theoretic identifiability conditions. Third, we derive the sample complexity of MMC. Finally, we give a practical algorithm for MMC with performance comparable to the state-of-the-art for simpler related problems, both on synthetic and real data.

Review:
This paper presents mixture matrix completion (MMC) as a novel machine learning tool for learning from low-rank and incomplete data. MMC is a problem that is similar to the problem of subspace clustering with missing data, but is more difficult. Specifically, in MMC the data is assumed to lie in a union of (unknown) low-dimensional subspaces, but the data is not fully observed: only a few entries of each data point are observed, and (unlike subspace clustering with missing data) there is no information as to which entries correspond to the same point. Therefore, one would need to estimate the assignment of entries to data points, the assignment of data points to subspaces, the missing entries, and the subspaces altogether. The major contribution of this paper is the introduction of the MMC problem, a theoretical analysis for when the problem of MMC is well-defined, and an alternating estimation algorithm for solving the MMC problem.  Strengths:  - The paper presents a new machine learning problem formulation that seems natural for addressing practical tasks such as background segmentation. It also has a nice preliminary study of this problem in terms of atheoretical analysis of the identifiability problem, a simple practical algorithm, and experiments on real and synthetic data.   - The paper is well written and the logic flow is clear.   Weaknesses:   - My major concern with the paper is that the theoretical results seem to be stated in vague terms and I don't fully understand them. In Theorem 1, what does it mean to say that Omega^k "contains" disjoint matrices Omega_tau? Does it mean that  Omega^k is a stack of matrices Omega_tau column-wise? Also, what does it mean that it is "possible" to perfectly recover all columns of X^k? Does it mean that the subspaces and the missing entries can be uniquely determined from the observed data? In addition, how is this result compared with the deterministic result for the problem of subspace clustering with missing entries in [4]?  Overall this is a nice paper that brings up a new problem formulation into attention. The study of this problem is still preliminary, though, as one can clearly see that there is no very successful application of the proposed method yet. The experiment on face clustering has a very unrealistic test scenario, and the experiment for background segmentation does not generate results as good as classical robust PCA. Nonetheless, addressing these challenges could be the topic of future study. My reason for not having a higher rating is that I cannot fully appreciate the theoretical studies as mentioned above.  Additional comments:  - It appears that the "cluster" step in the proposed algorithm is very complicated. I'm wondering if this step can be decomposed into the following substeps to make it easier to explain: the estimation of Omega_k is composed of two separate tasks of 1) clustering the entries in each column to different data points, and 2) the assigning data points extracted from all columns to multiple subspaces. In fact, once one have solved task 1) above then the problem reduces to subspace clustering with missing data, which could be solvedby alternating between matrix completion and subspace clustering.  - Since the overall problem is nonconvex, initialization is expected to be very important for the algorithm to achieve good performance.  Can the authors comment on how their algorithm is initialized on real data experiments?  Response to rebuttals:  The updated statement of Theorem 1 in the rebuttal is now much better in terms of clarity and it appears to be very necessary to incorporate it into the final version. Also, given that the conditions in Theorem 1 and 2 are very much similar to those in [4] for a related problem, a more detailed discussion of their connections will help understand the merits of these results. I maintain my overall rating as above and recommend a weak acceptance for this work.