REVIEW 2 major objections 3 minor 37 references
Two-Sided Nearest Neighbors: An adaptive and minimax optimal procedure for matrix completion
T0 review · 2 major / 3 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read A two-sided nearest-neighbor estimator achieves the nonparametric minimax optimal mean-squared-error rate for matrix completion under a Hölder-smooth latent factor model, without observing row or column latent factors, and the same rate…
desk verdict Plausible and important extension of two-sided nearest neighbors to Holder lambda<1, but the central MSE proof has a load-bearing gap in the B1 bias term, so the minimax claim is unproved as written. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the TS-NN estimator: it estimates pairwise row distances by averaging squared differences of observed entries across common columns (minus $2\sigma^2$), does the same for columns, thresholds these distances at $\eta_{\mathrm{row}}$ and $\eta_{\mathrm{col}}$ to form neighborhoods, and predicts $\hat{\theta}_{i,j}$ as the average of observed entries in the intersection of row $i$'s and column $j$'s neighborhoods. The proof rests on a distance-concentration lemma showing the estimated distances track their population counterparts, a bias–variance decomposition in which the bias is controlled by Hölder continuity and the variance by neighborhood size, and a sub-sampling step that caps neighborhood sizes so that Hoeffding-type noise concentration applies. Under MNAR, Assumption 6 replaces the MCAR observation probability $p$ by a lower bound $g(\delta)$ on the observed fraction of each neighborhood, which is shown via weighted Bernoulli concentration to hold in examples with up to roughly 50% deterministic missingness.
What would settle it
Run TS-NN on $f(u,v)=|u+v|^{0.5}\operatorname{sign}(u+v)$ with uniform latent factors, $n=m$, and an MNAR mask that sets $A_{i,j'}=0$ for a fixed target row $i$ at columns where most neighbor rows are observed while keeping Assumption 6 satisfied overall; if the pointwise error at $(i,j)$ or the population MSE decays at a rate strictly worse than $n^{-4\lambda/(2\lambda+2)}$, the bias replacement in Eq. (16) of Appendix A is doing real work and the stated rate is not a consequence of the stated assumptions.
Extended reading notes
Core claim
The paper's core discovery, stated as Corollary 1 and Theorem 2, is that TS-NN($\eta$) with $\eta_{\mathrm{row}} = \eta_{\mathrm{col}} = \Theta((mn)^{-\lambda/(2\lambda+d_1+d_2)})$ attains $\mathrm{MSE} = O((mn)^{-2\lambda/(2\lambda+d_1+d_2)})$ under the Hölder factor model, provided $n = \omega(m^{d_1/(2\lambda+d_2)})$ and $n = O(m^{(2\lambda+d_1)/d_2})$. The same rate persists under MNAR missingness when each neighborhood retains at least a constant fraction of observed pairs (Assumption 6), and the analysis is claimed to be the first to handle unknown row and column factors, Hölder smoothness with $\lambda < 1$, and missingness not at random simultaneously. At the entrywise level, Theorem 3 gives a slower pointwise rate with exponent $2\lambda/(2\lambda+d_1+d_2+d_1d_2/\lambda)$, and Theorem 4 gives asymptotic normality of each estimate, so that confidence intervals can be formed from the neighborhood average and an estimated noise level.
Load-bearing premise
The proof of the optimal rate assumes that averaging over a target row's neighbor rows can be reduced to the target row's own column differences and its own observed entries, so a neighbor row's column-to-column difference is no larger than the target row's and the target's observed entries proxy for the neighbors'; Hölder continuity does not imply this, and the replacement fails when the target entry is missing while its neighbors' entries are observed.
Editorial extensions
If this is right
- TS-NN matches the minimax nonparametric rate without observing $u_i$, $v_j$, or $f$, so in the moderate scaling regime simple two-sided averaging is statistically as good as knowing the latent structure.
- The MSE exponent $2\lambda/(2\lambda+d_1+d_2)$ varies continuously with the Hölder smoothness $\lambda$, so the estimator automatically benefits from smoother signals; simulations confirm steeper decay for $\lambda=0.75$ and $\lambda=1$ than for $\lambda=0.5$.
- Under Assumption 6, the same rate survives missingness that depends on the latent factors, including deterministic missingness of a substantial fraction of entries, which covers realistic recommender-system and micro-randomized-trial settings such as HeartSteps.
- Pointwise asymptotic normality means that uncertainty about a single entry can be reported as a confidence interval without refitting or bootstrapping, provided the noise level is consistently estimated.
- Because $\eta_{\mathrm{col}} \to 0$ recovers row-nearest-neighbor and $\eta_{\mathrm{row}} \to 0$ recovers column-nearest-neighbor, TS-NN is theoretically never worse than its one-sided counterparts.
Reading between the lines
- An implicit condition in the proof, not listed as an assumption, is that each target row's observed entries can serve as surrogates for its neighbors' observed entries; a testable extension is that the rate should degrade when missingness is concentrated on target rows while their neighbors stay observed, even under Assumption 6.
- The moderate scaling conditions $n = \omega(m^{d_1/(2\lambda+d_2)})$ and $n = O(m^{(2\lambda+d_1)/d_2})$ suggest that for strongly rectangular matrices the two-sided method may lose its edge and one-sided NN could be preferable; this is an extrapolation from the rate conditions, not a result proved in the paper.
- The same bias-variance machinery would likely extend to smoother function classes (for example, smooth reproducing-kernel-Hilbert-space functions) or to dependent latent factors, since the core argument only uses local Hölder control; the paper itself leaves these as open directions.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies two-sided nearest-neighbor (TS-NN) matrix completion under a non-linear latent factor model in which the latent function f is (λ,L)-Hölder with λ ≤ 1 and the row and column latent factors are unobserved. The main theoretical claims are a high-probability MSE bound for MCAR missingness (Theorem 1), an optimized rate matching the non-parametric minimax rate in a moderate scaling regime (Corollary 1), an extension to MNAR missingness under a lower-bound condition on the number of observed neighbors (Theorem 2), pointwise guarantees (Theorem 3), and asymptotic normality of the TS-NN estimate (Theorem 4). The paper also reports simulations and a HeartSteps case study. The central proof step used to control the B1 bias term in Theorem 1 is invalid, and the same step is reused in Theorem 2; as a result the main MSE optimality claims are not established as written.
Significance. If the main results were valid, the paper would be of clear interest: it claims adaptive minimax-optimal rates for TS-NN without knowledge of latent factors, handles Hölder smoothness below Lipschitz, and extends to MNAR settings with deterministic missingness. The paper is well organized, the algorithm is simple, and the simulation study is extensive. The paper also makes explicit, falsifiable rate predictions that are checked empirically. However, the central proof gap in the B1 bias bound is load-bearing: it is the step through which the optimized rates of Corollary 1 and the MNAR corollary are obtained. A secondary issue is that 'minimax optimal' is asserted without a stated or proved lower bound for the model class. These concerns outweigh the paper's strengths for the current version.
major comments (2)
- [Appendix A, proof of Theorem 1, Eq. (16), step (i)] The bound for the bias term B1 replaces the double sum over neighbor rows i' and neighbor columns j' of (f(i',j')-f(i',j))^2 A_{i',j'} by a term in which the row sum is collapsed to the target row i: the displayed step (i) effectively uses |N_row(i)| (f(i,j')-f(i,j))^2 A_{i,j'} in place of the sum over i' of the corresponding quantities. This replacement is not an identity and does not follow from Hölder continuity. The row-neighborhood condition bounds an average over columns of squared row deviations; it gives no pointwise control at a fixed column j'. Moreover, the missingness indicator is changed from A_{i',j'} to A_{i,j'}: if A_{i,j'}=0 while several neighboring rows have A_{i',j'}=1, the right-hand side can be zero while the left-hand side is positive, and no event in the proof rules out such a configuration. The Chernoff bounds used nearby control row or column marginals, not this per-cell replacement. The identical aggregation step is reused in the MNAR proof of Theorem 2 at Eq. (30). Since B1 is one of the two bias components through which the tuned rate MSE = O((mn)^(-2λ/(2λ+d1+d2))) is derived, Theorems 1 and 2 and Corollary 1 are not proved as written.
- [Corollary 1 and Section 6] The paper claims that TS-NN achieves the 'non-parametric minimax optimal rate' and uses the word 'minimax' in the title, but no lower bound is stated or proved for the model class defined by Assumptions 1-4 or Assumptions 1-3, 5, and 6. Matching the rate of an oracle that knows the latent factors is not by itself a minimax lower bound for the problem with unknown latent factors and Hölder f. The authors should either prove a matching lower bound for this model class or cite a precise existing lower-bound result and explain why it applies. Without this, the optimality claim in Corollary 1 and the discussion is not supported.
minor comments (3)
- [Appendix D, Lemma 3] The proof of Lemma 3 writes ||g||_∞ = B and assumes g(x*) = B for a point x*, but if the supremum of |g| is attained at a point where g is negative, the displayed argument 'g(x) ≥ g(x*) - L||x-x*||^λ' is not the correct starting point. This can likely be repaired by working with |g| or by a sign flip, and it is a local issue rather than a central one.
- [Appendix A, proof of Theorem 1] The appendix states that sample splitting is unnecessary and that 'all the computations will continue to hold' on a single dataset, but the proof conditions on events A1, A2, and A3 whose definitions involve the estimated neighborhoods, which depend on the same observations used in the final estimator. A short formal justification of this conditioning, or an explicit sample-splitting version of the proof, would make the argument easier to verify.
- [Acknowledgments] The Acknowledgments section contains the placeholder text 'Give acknowledgements' and should be completed before resubmission.
Circularity Check
No significant circularity: the MSE rates are derived from the stated assumptions, the cited lemmas from the authors' prior work are auxiliary concentration tools, and the minimax benchmark is external.
full rationale
I walked the derivation chain for Theorems 1--4 and Corollary 1. The claimed MSE rate O((mn)^(-2lambda/(2lambda+d1+d2))) is obtained by optimizing an explicit high-probability upper bound with respect to the tuning parameters eta_row and eta_col; the bound itself is not defined in terms of the target rate, so Corollary 1 is an algebraic consequence rather than a self-definitional equivalence. The MNAR result (Theorem 2) is conditional on Assumption 6, a stated sufficient condition on observed-neighborhood size, not a restatement of the conclusion. The minimax optimality language invokes the lower bound of Yu (2022), which is an external benchmark and is not proved or assumed circularly here. The self-citations to Dwivedi et al. (2022a,b) are used for auxiliary tools: weighted-Bernoulli concentration (Lemma 2 of Dwivedi et al. 2022b), a calculus inequality imitated from Lemma H.2 of Dwivedi et al. (2022a), and a variance-estimation suggestion in Appendix E.1. None of these supplies the main bias-variance trade-off or the minimax rate, so they are not load-bearing circularity. I also considered the skeptic's critique of Appendix A, Eq. (16), step (i), where the proof aggregates row-neighbor terms and replaces A_{i',j'} by A_{i,j'}; this is a possible correctness gap in the B1 bias bound, and the appendix's assertion that all computations hold without sample-splitting is an unproved claim. However, a proof gap is not the same as circularity: it does not make the theorem's conclusion equivalent to its inputs by construction. For circularity, the paper would need to fit or define the claimed rate into the assumptions, and I found no such reduction.
Assumptions & free parameters
free parameters (4)
- eta_row =
Θ((mn)^(-λ/(2λ+d1+d2))) in Cor. 1; tuned by 5-fold CV in experiments
- eta_col =
Θ((mn)^(-λ/(2λ+d1+d2))) in Cor. 1; tuned by 5-fold CV in experiments
- subsampling threshold tau =
tau > 1 (unspecified constant)
- T_n,m =
Sequence satisfying Assum. 7
assumptions (8)
- domain assumption Latent factor model with Holder function f (Assum. 1)
- domain assumption Sub-Gaussian noise independent of factors and missingness (Assum. 2)
- domain assumption Latent factors i.i.d. Uniform[0,1]^d (Assum. 3)
- domain assumption MCAR missingness (Assum. 4) or MNAR with independence (Assum. 5)
- domain assumption Minimum observed-neighbor fraction g(δ) (Assum. 6)
- domain assumption Neighborhood-size cap T_n,m (Assum. 7)
- standard math Uniform bound on noise: max |ϵ| ≤ σ√(2 log(2mn/δ))
- ad hoc to paper Unproven step: neighbor-row column-difference replacement (Appendix A step (i))
Cite this review
Pith. "Pith review of Two-Sided Nearest Neighbors: An adaptive and minimax optimal procedure for matrix completion." pith.science (2026). https://pith.science/paper/RIDDFFTC
@misc{pith2026241112965,
author = {Pith},
title = {Pith review of: Two-Sided Nearest Neighbors: An adaptive and minimax optimal procedure for matrix completion},
year = {2026},
howpublished = {\url{https://pith.science/paper/RIDDFFTC}},
note = {Machine review of arXiv:2411.12965}
}
read the original abstract
Nearest neighbor (NN) algorithms have been extensively used for missing data problems in recommender systems and sequential decision-making systems. Prior theoretical analysis has established favorable guarantees for NN when the underlying data is sufficiently smooth and the missingness probabilities are lower bounded. Here we analyze NN with non-smooth non-linear functions with vast amounts of missingness. In particular, we consider matrix completion settings where the entries of the underlying matrix follow a latent non-linear factor model, with the non-linearity belonging to a \Holder function class that is less smooth than Lipschitz. Our results establish following favorable properties for a suitable two-sided NN: (1) The mean squared error (MSE) of NN adapts to the smoothness of the non-linearity, (2) under certain regularity conditions, the NN error rate matches the rate obtained by an oracle equipped with the knowledge of both the row and column latent factors, and finally (3) NN's MSE is non-trivial for a wide range of settings even when several matrix entries might be missing deterministically. We support our theoretical findings via extensive numerical simulations and a case study with data from a mobile health study, HeartSteps.
Figures
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Reference graph
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Reviewed August 12, 2026 · model on record in the stance chip above.
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