NeurIPS 2020

Learning Some Popular Gaussian Graphical Models without Condition Number Bounds

Review 1

Summary and Contributions: The paper focuses on the problem of structure learning in Gaussian graphical models with additional constraints like attractiveness or walk-summability. It is shown that these special subclasses indeed enable efficient (polynomial-time) algorithms with superior sample complexity bounds. An important contribution of the paper is also providing some better intuition for which are the challenging examples for GGM structure recovery.

Strengths: The paper seems fairly well written and the math is convincing. I agree that the bounds on sample complexity should be ideally expressed only in terms of bounds on the partial correlations and the maximum degree. It is interesting to see that to large extent this is possible for attractive GGMs.

Weaknesses: Attractiveness is a very strong condition. To argue why the paper is a significant contribution to the literature, the authors should elaborate a bit more why this assumption is important from the practical point of view. Perhaps a good place to start is a recent paper: Agrawal, Raj, Uma Roy, and Caroline Uhler. "Covariance matrix estimation under total positivity for portfolio selection." arXiv preprint arXiv:1909.04222 (2019). Another weakness (but this is a growing problem overall) is that the whole interesting math is moved to the appendix and the paper became only a walk-through through the main ideas. I personally do not like this style as it is hard to get the feeling behind the structure of the problem without jumping back and forth between the paper and the appendix. Finally, below Lemma 1 you comment on other approaches to learning attractive GGMs. But you missed a paper in which actually some sample complexity results are provided: Yuhao Wang, Uma Roy, Caroline Uhler ; Proceedings of the Twenty Third International Conference on Artificial Intelligence and Statistics, PMLR 108:2698-2708, 2020. I think this has to be explicitly mentioned. This may involve some further changes in the paper. EDIT: I suggested motivating the paper by discussing where attractiveness appears because I felt this may be just very easy. But I actually agree with the fourth reviewer that it would be great to get a better feeling behind walk-summability. EDIT2: Thank you for clarification about the missing citation. Indeed, their result has a different flavour than yours. I felt this could be the only remaining issue but now I am happy to change my recommendation.

Correctness: I did not check the proofs carefully. I went through the first results in the appendix and it has a good flow so I generally believe in correctness of the paper.

Clarity: Yes, subject to some minor comments below.

Relation to Prior Work: Yes, I think the paper is really well linked to previous results (apart from my comment above).

Reproducibility: Yes

Additional Feedback: One page 4 you write that attractive GGMs are positively associated. I would try to be more precise because positive association has a meaning (Easy, Proschan,Walkup). You can write explicitly write that all partial correlations are nonnegative. On Page 4 the algorithm must be edited better. You must write that i is fixed. In Var(X_j|X_S,X_j) replace the first X_j with X_i. It is also not clear at what stage the value of t enters the picture. On page 6 there is "that that"

Review 2

Summary and Contributions: This paper gives the first fixed polynomial-time algorithms for learning attractive GGMs and walk-summable GGMs with a logarithmic number of samples when the precision matrix is ill-conditioned. The authors present the theoretical analysis of the proposed algorithms and empirically evaluate the effectiveness of the algorithms.

Strengths: The contribution is significant and novel: Under the assumption that the precision matrix is ill-conditioned, the recent work [14] provides an algorithm for learning GGMs with n^{O(d)} runtime which is inefficient. In this paper, the authors provide polynomial-time algorithms with theoretical guarantees for a class of GGMs under the same assumption. In addition, the proposed algorithms still have logarithmic sample complexity as [14]. They also prove information-theoretically optimal sample complexity for learning attractive GGMs, which previous literature leaves open. The experimental results show that the proposed methods can achieve much lower recovery errors than existing methods when the number of node is large given a fixed sample number.

Weaknesses: As this paper studies the runtime of the algorithms, the authors can also provide empirical comparison of the CPU time of different algorithms in the main text.

Correctness: The theoretical claims look correct. The detailed proofs are not carefully checked. The empirical methodology is correct.

Clarity: This paper is well written and well organized.

Relation to Prior Work: The authors have clearly compared their results with previous works.

Reproducibility: Yes

Additional Feedback:

Review 3

Summary and Contributions: The authors considered the problem of reconstructing the structure of gaussian graphical models (GGM) using an optimal number of samples. In particular they authors found subclasses of GGMs (walk-summable and positive GGMs) for which they designed low computational complexity algorithms that provably achieve optimal samples complexity.

Strengths: I believe this work is a very important theoretical contribution as GGM are the most fundamental probabilistic models and in my opinion these results are also quite relevant for practical applications (GGM are used everywhere in natural sciences where low sample complexity algorithm are often extremely important). I hope this paper will help the ML community to refocusing on finding optimal methods for learning arbitrary GGMs.

Weaknesses: This paper does not suffer from major weaknesses.

Correctness: The proofs look all correct to me.

Clarity: The paper is well-written, well-structured and easy to read.

Relation to Prior Work: I believe that the authors did an excellent job at putting their results into perspective and describing the previous contributions found in the litterature.

Reproducibility: Yes

Additional Feedback:

Review 4

Summary and Contributions: Setup. Fix kappa>0. Let Theta denote a square matrix such that - Theta is symmetric nonnegative positive definite - For every i,j with Theta_ij!=0, |Theta_ij|/sqrt(Theta_ii Theta_jj) > kappa. Given m i.i.d samples from N(0,Sigma), our task is to uncover the sparsity pattern of Theta. This paper develops a simple but effective algorithm to solve this problem, even if Theta is quite poorly conditioned.

Strengths: The method is simple, it appears to work, and it solves a problem that comes up all the time. The condition number problem is real: in practice two variables may be incredibly tightly correlated. It appears plausible that this approach works just as well in that case.

Weaknesses: Not all GPs are walk-summable. The end of the supplement gives some helpful intuition on what can go wrong. There's definitely still work to be done there to figure out how to know whether this algorithm is safe to apply...

Correctness: Looks plausible.

Clarity: Looks good. I would have included more simulations/experiments and buried more technical details in appendix. For example, all attractive gaussians are also walk-summable. So maybe you could just skip the in-depth discussion of the attractive gaussians. Mention them, but maybe not in so much detail. That would leave more room for showing some of the interesting examples where the method fails (because its not walk-summable). Dunno about other reviewers, but I always like to see at least some failure modes of a new method.

Relation to Prior Work: Seems clear.

Reproducibility: Yes

Additional Feedback: Minor -- the notation in line 136, I find "Var(Xj | Xs, Xj)" mysterious. In the supplement it is clear what you want to do, but the notation here makes it look like you want the variance of Xj given Xj... which is zero...