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Generalization of Hamiltonian algorithms
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Proves generalization bounds for stochastic Hamiltonian learning algorithms (Gibbs, randomized stable algorithms, data-dependent-prior PAC-Bayes) by bounding the log-moment generating function of the generalization gap via concentration of the log-density.
Problem
Generalization guarantees with high probability over both the sample and a single hypothesis drawn from a stochastic learning algorithm, including Gibbs posteriors and PAC-Bayes bounds with data-dependent priors.
Approach
Write the stochastic algorithm's density as exp(Hamiltonian)/partition function relative to an a-priori measure; bound the exponential moment of the generalization gap by showing the Hamiltonian (and its log-partition function) concentrates via bounded differences or subgaussianity, then apply Markov's inequality.
Key results
- For the Gibbs algorithm with bounded losses in [0,1], the bound gives Delta <= beta/n + sqrt(ln(1/delta)/(2n)) with probability 1-delta, removing log factors relative to prior Gibbs bounds.
- For Gaussian randomization of stable algorithms, the paper proves Delta <= sqrt(((3/sigma^2) V(A) + ln(1/delta))/n), with V(A) the variance of the algorithm output.
- A PAC-Bayes bound with data-dependent Gaussian prior centered at A(X) gives E_P[kl] <= (2KL(P,Q_X) + (6/sigma^2)V(A) + ln(2 sqrt n) + 2 ln(1/delta))/n.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use the Hamiltonian method for the Gibbs algorithm instead of the cited prior bound | Gibbs algorithm with bounded losses · Gibbs algorithm, bounded losses | generalization gap upper bound: \frac{4\beta }{n}+\frac{2+\ln ( (1+\sqrt{e})/\delta ) }{\sqrt{n}} -> \frac{\beta }{n}+\sqrt{\frac{\ln (1/\delta ) }{2n}} | Helped |
| Use the Gaussian-randomization bound based on algorithm variance instead of the cited stability-based PAC-Bayesian bound | Gaussian randomization of stable algorithms · Gaussian randomization of stable algorithms | expected KL generalization bound | Mixed |
| Use the disintegrated Gaussian-randomization bound | Gaussian randomization of stable algorithms · Gaussian randomization of stable algorithms | generalization gap upper bound | Helped |
Limitations
- Bounds for the Gibbs algorithm in the regime beta > n are not obtained; the authors say the method is unlikely to apply without strong assumptions.
- Constants increase when combining Hoelder/Cauchy-Schwarz to separate the contributions of F and H_Q.
- Results are stated for iid data; non-iid (Markov) extension is only proposed as future work.
Experimental setup
Metrics: generalization gap bound kl divergence bound log-moment generating function bound
Baselines: Rivasplata Gaussian PAC-Bayes bound Catoni Gibbs bounds kl-based Gibbs bounds
Builds on: PAC-Bayes theorem McDiarmid inequality Catoni PAC-Bayes Bernstein inequality
Closest papers by meaning
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