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Finite-class agnostic ERM bound

Statement

Let (X,ΣX) be a measurable space, let P be a probability law on X×{0,1}, and let H be a finite nonempty class of measurable classifiers h:X→{0,1}. Let Zi=(Xi,Yi), 1≤i≤m, be IID with law P, where m≥1, and define RP(h)=P{h(X)≠Y} and R^S(h)=m−1∑i=1m1{h(Xi)≠Yi}. With natural logarithms, for 0<δ<1 put

am,H,δ:=log⁡∣H∣+log⁡(2/δ)2m.

Then, with probability at least 1−δ, both ∣RP(h)−R^S(h)∣≤am,H,δ for every h∈H and

RP(h^η(S))−inf⁡h∈HRP(h)≤2am,H,δ+η,

for each fixed finite tolerance η≥0, where h^η is the measurable finite-list empirical-risk selector applied with tolerance η to an ordered list containing every member of H exactly once. In particular, exact ERM (η=0) has excess risk at most 2am,H,δ. The conclusion is agnostic: it assumes no zero-risk classifier. No form of the Axiom of Choice is assumed.

Statement references

Realizable and agnostic PAC learning: defines realizable and agnostic PAC learning for finite measurable binary classes.

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

Uniform deviation bound for a finite unit-range class: gives the choice-free two-sided tail bound for finite classes of measurable functions in [0,1].

The natural logarithm as the inverse of the exponential function: defines log⁡ as the inverse of exp⁡ on positive inputs.

Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm: establishes that log⁡ is increasing and obeys the product, quotient, and reciprocal laws.

Finite-list empirical risk minimization: specifies the deterministic finite-list ERM selector.

The finite-list ERM selector is measurable: proves that selector measurable and exact or approximate ERM.

Facts & Assumptions

Given: A measurable binary classification problem, a finite nonempty measurable classifier class H, IID observations Z1,…,Zm with law P, m≥1, 0<δ<1, and a finite tolerance η≥0.

[A1]

No form of the Axiom of Choice is assumed; the finite-list selector uses the given finite ordering, and the uniform-deviation supplier states that its proof is choice-free.

[F1]

For a finite nonempty class G of measurable functions g:Z→[0,1] and IID observations with law P, P{sup⁡g∈G∣Pmg−Pg∣≥t}≤2∣G∣e−2mt2 for every t>0; its proof uses scalar exponential moments and finite independence and does not assume choice. Uniform deviation bound for a finite unit-range class

[F2]

A supervised prediction problem has measurable instance and label spaces, a joint law on their product, a class of measurable predictors, and a measurable loss. Supervised prediction problems

[F3]

Population risk is the integral of the loss under the joint law, and empirical risk is the average loss on the ordered sample. With binary zero-one loss these are P{h(X)≠Y} and m−1∑i1{h(Xi)≠Yi}. Population and empirical risk

[F4]

An η-approximate ERM has empirical risk at most the empirical infimum plus η when that infimum is finite; an exact ERM attains the infimum. Empirical risk minimizers

[F5]

On a finite ordered list with finite empirical risks, the finite-list procedure returns the first list entry whose empirical risk is at most the minimum plus η; at η=0 it returns a first exact minimizer. Finite-list empirical risk minimization

[F6]

For a finite ordered list of measurable predictors and finite η≥0, the finite-list selector is measurable and is an η-approximate ERM; when η=0 it is an exact ERM. The finite-list ERM selector is measurable

[F7]

IID means the observations are mutually independent and identically distributed. Identical distribution and IID families

[F8]

The natural logarithm is increasing, log⁡1=0, and obeys the product, quotient, and reciprocal laws; it is the inverse of the exponential on positive reals. The natural logarithm as the inverse of the exponential function, Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm

Proof

Given: The hypotheses in the Statement.

1.1givenF2F3F7A1construct

For each h∈H, define gh(x,y)=1{h(x)≠y}. Its unit-level set is (h−1{0}×{1})∪(h−1{1}×{0}), a measurable union of rectangles because h is measurable and {0,1} is discrete. Thus gh is measurable and [0,1]-valued; G:={gh:h∈H} is finite and nonempty with ∣G∣≤∣H∣. By [F3], Pgh=RP(h) and Pmgh=R^S(h).

2.1F1F7F8step 1.1A1algebra

By [F7], the observations meet the IID hypothesis of [F1]. Put a=am,H,δ. Since ∣H∣≥1 and 2/δ>1, [F8] makes the numerator positive, so a>0. Apply [F1] to G at t=a. By [F8], log⁡∣H∣+log⁡(2/δ)=log⁡(2∣H∣/δ) and e−log⁡(2∣H∣/δ)=δ/(2∣H∣). Hence 2∣G∣e−2ma2≤2∣H∣e−2ma2=δ. With probability at least 1−δ every h∈H satisfies ∣RP(h)−R^S(h)∣<a, hence also the asserted non-strict bound.

3.1F3F4F5F6step 2.1algebra

On that event, [F6] gives R^S(h^η)≤inf⁡h∈HR^S(h)+η. The deviation bound gives RP(h^η)≤R^S(h^η)+a and R^S(h)≤RP(h)+a for every h. Hence inf⁡g∈HR^S(g)≤RP(h)+a for every h∈H, and taking the infimum over h gives inf⁡g∈HR^S(g)≤inf⁡h∈HRP(h)+a. Combining these inequalities gives RP(h^η)−inf⁡h∈HRP(h)≤2a+η.

4.1step 2.1step 3.1∎

The event from step 2.1 has probability at least 1−δ, and step 3.1 establishes both stated inequalities on that event. This proves the claim for arbitrary P, whether or not the class is realizable.

Depends on

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