Paper:
Representation Learning of Compositional Data
We consider the problem of learning a low dimensional representation for compositional data. Compositional data consists of a collection of nonnegative data that sum to a constant value. Since the parts of the collection are statistically dependent, many standard tools cannot be directly applied. Instead, compositional data must be first transformed before analysis. Focusing on principal component analysis (PCA), we propose an approach that allows low dimensional representation learning directly from the original data. Our approach combines the benefits of the log-ratio transformation from compositional data analysis and exponential family PCA. A key tool in its derivation is a generalization of the scaled Bregman theorem, that relates the perspective transform of a Bregman divergence to the Bregman divergence of a perspective transform and a remainder conformal divergence. Our proposed approach includes a convenient surrogate (upper bound) loss of the exponential family PCA which has an easy to optimize form. We also derive the corresponding form for nonlinear autoencoders. Experiments on simulated data and microbiome data show the promise of our method.

Review:
This paper suggests a generalization of PCA that is applicable to compositional data (non-negative data where each sample sums to 1 or to 100%). It builds up on exponential family PCA developed in the early 2000s. There have been multiple more or less ad hoc approaches of transforming compositional data to make it amenable to PCA, and a more principled method of compositional PCA would certainly be an interesting development.  The paper is reasonably well written and uses interesting mathematics. However, it (a) lacks convincing examples, (b) is somewhat confusing in places. I put it above the acceptance threshold, hoping that the authors would be able to improve the presentation.  Caveat: I am not familiar with Bregman divergences and do not know the [22] paper that this one heavily uses. I cannot therefore judge on the details of the math.  MAJOR ISSUES  1. When reading the paper, I was waiting for some cool visualizations of real-life data. I was disappointed to find *no* visualizations in Section 5.2. Actually the text says "visualizations of the two main axes..." (line 233 and 246), but they are not shown!! Please show! Also, you seem to be saying that all methods gave similar results (e.g. line 236). If so, what's the point of using your method over more standard/simple ones? I would like to see some example where you can show that your method outperforms vanilla PCA and alr/clr/ilr+PCA in terms of visualization. Your metrics in Figure 2 are not very convincing because it's hard to judge which of them should be most relevant (maybe you should explain the metrics better).  2. The toy example (Figure 1) is good, but I would like to see alr+PCA and ilr+PCA in addition to clr+PCA (I know that different people recommend different choices; does it matter here?), and more importantly, I'd like to see vanilla PCA. It seems to me that at least for 3-arms vanilla PCA should give an ideal "Mercedes" shape without any log-transforms. Is it true for 10-arms? If it _is_ true, then this is a poor example because vanilla PCA performs optimally. If so, the authors should modify the toy example such that alr/clr/ilr+PCA performed better than PCA (this would also help motivating clr-transform) and that CodaPCA performed even better than that.  3. I think a better motivation is needed for log-transforms in Section 2. Line 52 mentions "nonlinear structure due to the simplex constraint" but simplex constraint is clearly linear. So if the simplex constraint were the only issue, I wouldn't see *any* problem with using standard vanilla PCA. Instead, there arguably *is* some nonlinearity in the nature of compositional data (otherwise why would anybody use log-transforms), and I remember Aitchinson showing some curved scatterplots on the 2D simplex. This needs to be explained/motivated here.  4. Line 91 says that count data can be described with Poisson distribution. Line 260 again mentions "raw count data". But compositional data does not need to be count, it can come as continuous fractions (e.g. concentrations). Does the method suggested here only apply to counts? If so, this should be made very clear in the title/abstract/etc. If not, please clarify why Poisson distribution still makes sense.  MINOR ISSUES  1. Box after line 81. Shouldn't Eq 3 be part of the box? Without it the box looks incomplete. The same for the box after line 107.  2. Eq 6. "\phi-PCA" should be written with a hyphen (not minus) and capitalized PCA. Please use hyphens also below in similar expressions.  3. The end of Section 3 -- I suggest to insert some comments on how exactly exp family PCA reduces to standard PCA in the Gaussian case. What is \phi function, what is Bregman divergence, etc.   3. Line 113, last sentence of the section. This is unclear. Why is it a hole? Why is not easy?  4. Line 116, first sentence of the section: again, not clear how exactly clr is a workaround.  5. Line 139: period missing.  6. Line 144, where does this \phi() come from? Is it Poisson? Please explain.  7. Eq (11): what is "gauged-KL", the same as "gauged-phi" above?  8. Box after line 150: now you have loss function in the box, compare with minor issue #1.  9. Lines 201 and 214: two hidden layers or one hidden layer?  10. Line 206: space missing.  11. Line 211: isn't CoDA-PCA and SCoDA-PCA the same thing? Got confused here.  12. Line 257: I don't see any overfitting in Figure 2. This would be test-set curve going upwards, no? Please explain why do you conclude there is overfitting.  13. Shouldn't you cite Aitchinson's "PCA of compositional data" 1983?  -----------------  **Post-rebuttal update:** I still think the paper is worth accepting, but my main issue remains the lack of an obviously convincing application. I see that reviewer #3 voiced the same concern. Unfortunately, I cannot say that the scatter plots shown in the author response (panels A and B) settled this issue. The authors write that panel A, unlike B, shows three clusters but to me it looks like wishful thinking.  I am not familiar with the microbiome data but I do agree with the point of reviewer #3 that with 100+ features the compositional nature of the data can simply be ignored without losing much. Would the authors agree with that? I think it would be worth discussing.  Overall, it looks like a good attempt at setting up composional PCA in a mathematically principled way, but in the end I don't see examples where it performs noticeably better than standard naive approaches.