NeurIPS 2020

Simple and Scalable Sparse k-means Clustering via Feature Ranking

Review 1

Summary and Contributions: The paper is about unseupervised learning and the authors proposed sparse k-means clustering called SKRF. The authors claimed that SKRF is scalable and easy to implement. SKRF is evaluated on both synthetic and real data sets. However, my qesttiion is why authors didn't compare their method with stronger baselines? The selection of baselines needs revision. For instance, why authors didn't compare SKRF with reference [37] which has been published in ICML recently? +++++++++++++++++++++ After reading the author's response, I vote for acceptance since the authors addressed my misunderstanding.

Strengths: The paper is well-written and easy to understand. The topic is relevant to NeurIPS communitty but the contribution of the paper is limited. Authors need to compare their method with other strong baselines. Also they need to mentioned what is the benefit of SKRF over low-rank approximation like matrix or tensor factorization techniques.

Weaknesses: The contribution of this paper is very incremental and not enough for Neurips conference. Authors could pick better baselines for comparison. For instance, why they didn't compare their method with [37] "Power k-Means Clustering' which is recently published in ICML. Moreover, it is not clear the benefit of proposed technique over matrix and tensor factorization approaches?

Correctness: Minor issue: Capttion of Figure 3: feattures -> features

Clarity: The paper is well-written but the contribution is limited and not enough for Neurips Conference.

Relation to Prior Work: No, the difference with previiou works is not clear. Authors need to compare their proposed method with recent baselines.

Reproducibility: Yes

Additional Feedback:

Review 2

Summary and Contributions: This paper focuses on the problem of clustering in high dimension. K-means clustering is an extremely popular tool (especially in biomedical applications). However, as underlined by the authors, its performance is severely hindered in high-dimensional space --- leaving the data analyst no chance but to (a) apply some dimensionality reduction technique before performing the clustering or (b) selecting the features that are the most informative for the clustering and apply k-means on a subset of the features. This paper proposes a version of the later approach, choosing a sparse and interpretable subset of features. The setting is the following. Consider a dataset of d-dimensional observations (variables have been standardized.). The authors compute a score for each variable, which is basically the total variance of the observed values for that variable within each cluster. The $s$ variables with lower scores are used to assign variables to clusters (standard Lloyd’s step). In a way, this score is indicative of which variable achieve better clustering (less spread out cluster) as a measure of the relevance of the observations. The authors then prove the consistency of their added step with standard clustering, the convergence of their algorithm to the optimum (under the constraint that the features have been properly selected) and compare their method to other feature-selection methods for clustering .

Strengths: 1) Extremely simple and scalable algorithm: the solution proposed by the authors is simple, scalable, and intuitive. Its simplicity also allows it to benefit from theoretical guarantees for kmeans. 2) Ease of use: because $s$ is specified, we are ensured to select $s$ number of variables -- as opposed to using l1 penalty, where the effect of the penalty on the number of features selected is less obvious 3) Interpretable: since we are selecting features, (and not linear combination of features or lower embeddings, etc), the features that are selected are interpretable. This is great for biological applications, where we really wish to understand which features cause the clustering. 4) Robustness: Problems that arise in real data are tackled: the authors show the ease of adaptation of their method to the issues that plague real data analysis (missing data, outliers) 5) Experiments are well designed: the experiments that are led in this paper are very convincing --- both testing the approach on simulated data and on real data, reporting the standard deviation.

Weaknesses: Very convincing, theoretically sound, and potentially very useful paper, I found no real weakness. Maybe, it would have been interesting to discuss the use of their score (a sort of within-cluster sum of squares) compared to other metrics for computing the relevance of the features (like the entropy, mentioned in prior work), etc.

Correctness: Yes.

Clarity: Very well (see comment above)

Relation to Prior Work: Very clearly.

Reproducibility: Yes

Additional Feedback:

Review 3

Summary and Contributions: In this work, the authors describe a simple algorithm for sparse k-means clustering using feature ranking. The method is simple, generic and seems to provide a nice addition to the set of k-means related algorithms.

Strengths: The manuscript is well written and presents a simple but generic procedure to combine feature selection with K-means clustering. The authors showcase the strengths of their method on a number of applications.

Weaknesses: The major weak part of the paper is the experimental part. There, some more explanations about the the different datasets used would be welcome, as well as comparisons to other unsupervised feature selection methods.

Correctness: Claims are mostly correct, as well as the methodology.

Clarity: Very well written paper, easy to read.

Relation to Prior Work: While the authors discuss other related work, I am missing some comparisons to other unsupervised feature selection techniques (e.g. COSA). It would also be nice if some comparisons to purely filter-based unsupervised feature selection methods could be compared to.

Reproducibility: Yes

Additional Feedback: The authors claim the sorting step can be done in O(n) time, but this should be O(n log n)

Review 4

Summary and Contributions: This paper modifies the k-means objective by sorting features in a rank order such that the distance from 0 is maximized while the cluster membership distance is minimized. By doing so, the algorithm has a controllable sparsity. Two variants are presented, one that selects features based on all clusters and another that selects features per cluster.

Strengths: I enjoyed this paper. There are proofs in the supplement and the empirical evaluation is there. The discussion of extending the method to other projection operators is also interesting. This paper seems relevant to the community.

Weaknesses: One possible weakness is the claim that the selected features are more interpretable (282). Perhaps a table comparing some features selected by L1 vs ranking could be shown as an example to the reader how they are more interpretable vs domain specific examples like proteins. Maybe from a text corpus where k-means has traditionally been terrible because of the sparse high dimensionality of the features. The other use of the word interpretable when referring to the sparsity structure is also overloaded. Perhaps controllable sparsity structure is a better term. I have no idea what interpretable sparsity structure even means and it conflates the two uses of the word.

Correctness: The metric of NMI and the data sets used seem reasonable to me.

Clarity: Yes, I enjoyed this paper. It was written clearly and can be understood by many attendees of the conference.

Relation to Prior Work: Yes, the authors compare previous work and specifically compare to an L1 lasso version of the algorithm vs their ranking method.

Reproducibility: Yes

Additional Feedback: Post rebuttal I still like the paper and stick to my score