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ERM excess risk from Rademacher uniform deviation

Statement

Assume AC. Let (X,ΣX), (Y,ΣY), and (A,ΣA) be nonempty measurable spaces, let P be a probability measure on X×Y, let H be a nonempty class of measurable predictors h:X→A, and let ℓ:A×Y→[0,1] be measurable. For h∈H, define gh(x,y):=ℓ(h(x),y), G:={gh:h∈H}, and G‾:={1−g:g∈G}. Assume the sample, independent-copy, and sign suprema over G and G‾ required by the uniform generalization theorem are measurable in their respective product sigma algebras. Let Zi=(Xi,Yi) be mutually independent with common law P, with m≥1, and write

RP(h):=Pgh,R^S(h):=Pmgh.

Suppose a measurable prediction rule h^η exists such that h^η(S)∈H for every sample S and each samplewise output is an η-approximate empirical risk minimizer, meaning R^S(h^η(S))≤inf⁡h∈HR^S(h)+η for a fixed finite η≥0. Assume S↦RP(h^η(S)) is measurable. For δ∈(0,1), with independent uniform signs σi, R^S(G)=Eσsup⁡g∈Gm−1∑iσig(Zi) and Rm(G)=ESR^S(G), each of the following excess-risk bounds holds with probability at least 1−δ:

RP(h^η(S))−inf⁡h∈HRP(h)≤4Rm(G)+2log⁡(2/δ)2m+η,

RP(h^η(S))−inf⁡h∈HRP(h)≤4R^S(G)+6log⁡(4/δ)2m+η.

The two displayed bounds are separate probability guarantees. This corollary asserts no existence of an empirical-risk-minimizing measurable rule for a general class.

Statement references

Supervised prediction problems: supplies measurable observation, prediction, and loss spaces with a joint law.

Identical distribution and IID families: IID means the sample coordinates are mutually independent and have the common law P.

Population and empirical risk: defines population risk as expected loss and empirical risk as its sample average.

Empirical risk minimizers: defines exact and η-approximate ERM rules and does not assert a selector exists for every class.

Population and empirical risks are measurable: proves measurability of induced losses and fixed-predictor risks.

Rademacher uniform generalization bounds: supplies the two separate two-sided uniform deviation bounds under AC.

One-sided Rademacher complexity: defines the one-sided empirical and expected complexities used in the conclusion.

The Axiom of Choice: AC is carried because the concentration-based supplier assumes it.

The natural logarithm as the inverse of the exponential function: defines the logarithm appearing in the confidence terms.

Mohri et al., Theorem 3.3 and complete proof, Chapter 3 §3.1: supplies the uniform Rademacher deviations; the two-deviation ERM comparison is proved locally.

Bartlett and Mendelson, Theorem 8 and complete proof, printed pp. 466–467: gives an independent uniform risk-bound argument in a different normalization.

Facts & Assumptions

Given: A supervised prediction problem with bounded measurable loss, IID sample, AC, and an existing measurable η-approximate ERM rule with h^η(S)∈H for every sample S and measurable selected population risk.

[A1]

Assume AC. It is used by the concentration theorem in the uniform-deviation supplier. The Axiom of Choice

[F1]

A supervised problem has nonempty measurable instance, label, and action spaces; a joint law; measurable predictors; and a measurable loss. Supervised prediction problems

[F2]

Population risk is the integral of the loss under P and empirical risk is its sample average. For gh(x,y)=ℓ(h(x),y) these equal Pgh and Pmgh. Population and empirical risk

[F3]

An η-approximate ERM relative to H belongs to H and has empirical risk at most the empirical infimum plus η when the infimum is finite; the supplied measurable prediction rule has h^η(S)∈H for every sample S, is jointly measurable in sample and new instance, and the definition asserts no selector exists for every class. Empirical risk minimizers

[F4]

The induced loss gh is measurable, and fixed-predictor population and empirical risks are well-defined. Population and empirical risks are measurable

[F5]

With probability at least 1−δ, sup⁡g∈G∣Pg−Pmg∣≤2Rm(G)+log⁡(2/δ)/(2m). Rademacher uniform generalization bounds

[F6]

With probability at least 1−δ, sup⁡g∈G∣Pg−Pmg∣≤2R^S(G)+3log⁡(4/δ)/(2m). Rademacher uniform generalization bounds

[F7]

The sample coordinates are mutually independent and all have common law P. Identical distribution and IID families

Proof

Given: The hypotheses in the Statement.

1.1F1F2F4F5F6F7givenconstruct

For each h∈H, the supervised loss function gh is measurable by [F1] and [F4]. By [F2], RP(h)=Pgh and R^S(h)=Pmgh, so the uniform bounds in [F5]–[F6] control all predictors in H under the IID hypothesis [F7].

2.1F2F3F5step 1.1algebra

On the event in [F5], put d:=2Rm(G)+log⁡(2/δ)/(2m). Then every h satisfies RP(h)≤R^S(h)+d and R^S(h)≤RP(h)+d. By [F3], R^S(h^η)≤inf⁡hR^S(h)+η. Since h^η(S)∈H by [F3], applying the first inequality to h^η and taking the infimum in the second gives RP(h^η)−inf⁡hRP(h)≤d+inf⁡hR^S(h)+η−inf⁡hRP(h)≤2d+η, where inf⁡hR^S(h)≤inf⁡hRP(h)+d.

2.2F2F3F6step 1.1algebra

On the event in [F6], put d:=2R^S(G)+3log⁡(4/δ)/(2m). The two inequalities RP(h)≤R^S(h)+d and R^S(h)≤RP(h)+d hold for every h. Since h^η(S)∈H, [F3] permits applying the first inequality to the selected predictor and gives RP(h^η)−inf⁡hRP(h)≤d+inf⁡hR^S(h)+η−inf⁡hRP(h)≤2d+η, using inf⁡hR^S(h)≤inf⁡hRP(h)+d.

3.1A1F5F6step 2.1step 2.2algebra∎

The events in [F5] and [F6] are the AC-dependent concentration branch [A1]. Each has probability at least 1−δ; step 2.1 proves the first displayed excess bound on the [F5] event, and step 2.2 proves the second on the [F6] event. Substituting each value of d from those steps gives respectively 4Rm(G)+2log⁡(2/δ)/(2m)+η and 4R^S(G)+6log⁡(4/δ)/(2m)+η. The guarantees are separate.

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