REVIEW 2 major objections 4 minor 29 references
PAC-Bayes with Backprop
T0 review · 2 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read The paper claims that backprop-minimized PAC-Bayes bounds on MNIST yield 1.4% test error together with a 2.3% risk certificate, an order of magnitude tighter than previous non-vacuous bounds.
desk verdict The 2.3% MNIST bound is not a valid PAC-Bayes certificate because the prior scale was picked by grid search on the training data; the rest of the paper is correct but standard. 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 objects are two PAC-Bayes upper bounds on the average risk $Q[L]$ of a randomized weight distribution $Q$. The PAC-Bayes-quadratic bound (Theorem 1, Eq. (6)) comes from inverting a binary-KL inequality and solving a quadratic inequality for the square root of $Q[L]$; the PAC-Bayes-$\lambda$ bound (Theorem 2, Eq. (7)) follows from the inequality $\sqrt{ab}\le(\lambda a + b/\lambda)/2$ and is minimized by alternating over $\lambda$ and $Q$. Both bounds are functions of the empirical risk, the KL divergence to a fixed data-free prior $Q_0$, the sample size $n$, and the confidence $\delta$. To make the objectives trainable by backprop, the paper uses the pathwise gradient estimator to differentiate through samples drawn from Gaussian or Laplace posteriors centered at the randomly initialized weights.
What would settle it
Fix every prior hyperparameter, including the prior scale, before seeing the MNIST labels, run the same PBB training, and recompute the risk bound; if the certificate is materially above 2.3%, the reported number depends on data-driven prior selection rather than on the training method.
Extended reading notes
Core claim
On the paper's own terms, the central claim is that PAC-Bayes with Backprop (PBB) works: minimizing the two objectives in Eqs. (6) and (7) with SGD produces a stochastic neural network whose average risk $Q[L]$ is both small and certified. The headline numbers are a test-set error near 0.014 on MNIST and a simultaneous non-vacuous upper bound near 0.023, compared with the previous best non-vacuous bound of 0.21 from data-dependent priors; on Binary MNIST the paper reports 0.015 test error with a 0.022 bound. The authors conclude that fixed data-free priors and standard SGD may suffice for self-bounding learning on MNIST, and they report similarly close bound/error pairs on five UCI data sets, while conceding that initial CIFAR-10 experiments gave certificates about three times the test error.
Load-bearing premise
The 2.3% guarantee holds only if the prior distribution, including its scale, is fixed without using the training data; the reported run was chosen from a grid sweep that picked the best bound on that same data.
Editorial extensions
If this is right
- If the central claim is right, a single training run on the available data produces both a neural network and a high-probability guarantee on its true risk, so the test set is no longer required to certify generalization.
- A certificate near the test error means PAC-Bayes bounds can be used as practical model-selection and early-stopping criteria, not just as theoretical afterthoughts.
- On MNIST, the data-dependent-prior machinery used in earlier work is not necessary for tight certificates; plain SGD with fixed priors reaches the same goal.
- The UCI and Binary MNIST results suggest the bound/error gap stays small on small and mid-size problems, while the CIFAR-10 gap shows the method has not yet scaled to large natural-image tasks.
Reading between the lines
- A fair re-test that fixes the prior scale before seeing the training data would likely loosen the reported 2.3% value, because the paper's grid sweep selected the best bound over that same data; this is testable and should be done before treating the certificate as valid.
- The KL term is the main obstacle on larger networks, so structured posteriors, weight coupling, or hierarchical priors are the natural next levers for extending PBB to CIFAR-10-class problems.
- Because the same machinery emits a certificate for any posterior, PBB could be used to fine-tune or compress an already trained network while producing a fresh, valid bound.
- The near-equality of bound and test error suggests the bound itself could serve as the training signal for architecture search, provided the prior is kept data-free.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes two PAC-Bayes training objectives for probabilistic neural networks: the objective flamb in Eq. (7), derived from the PAC-Bayes-lambda bound of Thiemann et al., and fquad in Eq. (6), derived from a quadratic inversion of the PAC-Bayes-kl bound. Both objectives are minimized by backpropagation-style gradient estimates. The authors report experiments on MNIST and five UCI datasets, claiming test error around 1.4% on MNIST together with non-vacuous risk upper bounds around 2.3%, which they describe as much tighter than earlier results by Dziugaite and Roy. They argue that these results suggest 'self-bounding learning' with fixed, data-free priors and classical SGD, without a data-splitting protocol.
Significance. If the reported certificates were valid, the paper would be a significant step in PAC-Bayes training of neural networks: it would show that a simple method using classical SGD and fixed priors can produce both competitive predictions and non-vacuous risk guarantees, and it would provide a useful baseline against methods that rely on data-dependent priors or specialized optimizers. The theoretical derivations of Theorems 1 and 2 appear mathematically sound under the stated assumptions, and the connection to the Bayes-by-Backprop objective is informative. However, the central empirical claim depends on the validity of the reported risk certificates, and that validity is undermined by the data-dependent selection of the prior scale and other hyperparameters. The significance is therefore conditional on repairing the experimental protocol.
major comments (2)
- [Section 6, Section 6.1] The reported PAC-Bayes certificate is voided by data-dependent prior-scale selection. The paper states that 'we performed a grid sweep over all the hyper parameters and then selected the run with the best risk upper bound,' and the swept hyperparameters include the prior distribution scale parameter with standard deviation values in [0.1, 0.09, ..., 1e-3], as well as pmin, learning rate, and momentum. Theorems 1 and 2 require Q0 to be a fixed, data-free distribution. A prior scale selected by minimizing the bound on the same training data is data-dependent, so the high-probability statement no longer holds for the selected run. Section 6.1 defends only the randomness of weight initialization as external to the training process; it does not address the selection of the prior scale or the selection of the best run. No union bound over the configuration grid is given. Consequently, the 2.3% bound in Table 1 is not a valid PAC-Bayes risk certificate, and the comparisons in Tables 1 and 2 do not support the paper's headline claim of tighter valid bounds.
- [Abstract, Section 7] The claim of 'self-bounding learning' is unsupported by the experimental protocol. The abstract explicitly frames the contribution as obtaining competitive test error and simultaneously computing a valid risk bound 'with no need to follow a data-splitting protocol.' That framing relies entirely on the prior being data-free. Since the prior scale and the reported run are selected using the training data, the bound is not a valid high-probability guarantee, and the conclusion in Section 7 that PBB 'output models that come with a performance guarantee' is not justified. A correct protocol would require fixing the prior before seeing the training data, using a separate validation set for hyperparameter selection, or applying a union bound over the grid; none of these is present.
minor comments (4)
- [Section 3] There is a typo in the sentence 'there are many other PC-Bayes bounds available'; this should read 'PAC-Bayes bounds.'
- [Section 4] The boundedness of the cross-entropy loss is not fully specified. Lower-bounding network probabilities by pmin makes the loss finite but does not by itself place it in the interval [0,1] required by Theorems 1 and 2; if the bound is evaluated for the cross-entropy loss, a normalization step or explicit statement is needed, and if the reported bounds are for the zero-one loss, this should be stated clearly.
- [Section 6, Eq. (3)] The inversion step leading to Eq. (3) uses the inequality kl(p||p') >= (p'-p)^2/(2p'), which is stated to be valid for p' < p, but this side condition is not mentioned in the text surrounding Eq. (3).
- [Section 6.2] The paper does not specify how the plotted and tabulated 'risk upper bound' is computed at the end of training: which loss function, which value of delta, and whether the bound is evaluated at the last iterate or at the iterate with the minimal bound. This information is needed for reproducibility.
Circularity Check
Reported 2.3% MNIST bound is a post-selection minimum over data-chosen prior scales, not a valid PAC-Bayes certificate.
-
fitted input called prediction
[Section 6, 'In all the experiments...' and Section 6.1; Eqs. (4) and (5)]
"In all the experiments on various data sets we performed a grid sweep over all the hyper parameters and then selected the run with the best risk upper bound. ... We did a grid sweep over the prior distribution scale parameter with standard deviation values in [0.1, 0.09, 0.08, 0.07, 0.06, 0.05, 4e-2, 3e-2, 2e-2, 1e-2, 5e-3, 1e-3]. The variance of the posterior distribution was initialized to the same value as the prior distribution variance."
The reported 0.023 risk upper bound is not a bound guaranteed by Theorems 1 and 2, because those theorems require a fixed, data-free prior Q0. Here the prior scale was selected by minimizing the same PAC-Bayes bound on the same training data, and the best run over the grid was then reported as the certificate. Section 6.1 defends only the randomness of weight initialization as 'data-free'; it does not address the data-dependent choice of the prior scale or the selection of the best of many runs. Without a union bound over grid configurations, the minimum of many data-dependent bounds is not a valid high-probability statement.
full rationale
The derivations of the two training objectives themselves are self-contained: Eq. (4) is obtained by solving the quadratic inequality (3), and Eq. (5) is the PAC-Bayes-lambda bound of Thiemann et al.; neither bound has fitted constants, and no self-citation chain is load-bearing. Test error values are measured on a held-out test set, so the 1.4% error estimate is not circular. The circularity is confined to the central certificate claim: the prior scale hyperparameter is chosen by sweeping over values and picking the run with the smallest bound on the training data, and that selected bound is then reported as a valid risk certificate. This makes the reported 2.3% bound a post-selection minimum rather than a certificate satisfying the data-free-prior condition of Theorems 1 and 2. Consequently, the paper's 'self-bounding learning' conclusion is partially circular in the sense that the predicted certificate is the output of an optimization over certificates. The mathematical bound derivations are not circular, so the score is not higher.
Assumptions & free parameters
free parameters (4)
- Prior scale parameter (sigma) for Gauss/Laplace prior =
Selected from grid [0.1, 0.09, 0.08, 0.07, 0.06, 0.05, 4e-2, 3e-2, 2e-2, 1e-2, 5e-3, 1e-3] by best risk upper bound
- Learning rate =
Selected from grid [1e-3, 5e-3, 1e-2]
- Momentum =
Selected from grid [0.95, 0.99]
- p_min (minimum probability clamp) =
Selected from grid [1e-2, 1e-3, 1e-4, 1e-5, 1e-8, 1e-16]
assumptions (3)
- standard math PAC-Bayes-kl theorem (Langford-Seeger/Maurer)
- standard math Inequality kl(p_hat || p) >= (p - p_hat)^2 / (2p) for p_hat < p
- domain assumption Prior centered at randomly initialized weights satisfies the data-free requirement
Cite this review
Pith. "Pith review of PAC-Bayes with Backprop." pith.science (2026). https://pith.science/paper/PQMV4KFB
@misc{pith2026190807380,
author = {Pith},
title = {Pith review of: PAC-Bayes with Backprop},
year = {2026},
howpublished = {\url{https://pith.science/paper/PQMV4KFB}},
note = {Machine review of arXiv:1908.07380}
}
read the original abstract
We explore the family of methods "PAC-Bayes with Backprop" (PBB) to train probabilistic neural networks by minimizing PAC-Bayes bounds. We present two training objectives, one derived from a previously known PAC-Bayes bound, and a second one derived from a novel PAC-Bayes bound. Both training objectives are evaluated on MNIST and on various UCI data sets. Our experiments show two striking observations: we obtain competitive test set error estimates (~1.4% on MNIST) and at the same time we compute non-vacuous bounds with much tighter values (~2.3% on MNIST) than previous results. These observations suggest that neural nets trained by PBB may lead to self-bounding learning, where the available data can be used to simultaneously learn a predictor and certify its risk, with no need to follow a data-splitting protocol.
Figures
Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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