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Expected generalization from uniform stability

Statement

Let (X,ΣX) and (Y,ΣY) be nonempty measurable instance and label spaces, and equip Z=X×Y with ΣZ=ΣX⊗ΣY. Let P be a probability measure on (Z,ΣZ), let F be a nonempty class of measurable functions f:(X,ΣX)→(R,B(R)), and let ℓ:R×Y→[0,∞) be measurable with respect to B(R)⊗ΣY. For f∈F and z=(x,y) write Lf(z)=ℓ(f(x),y), RP(f)=∫ZLf(z) P(dz), and R^S(f)=m−1∑i=1mLf(Zi). Let S=(Z1,…,Zm) have product law Pm on (Zm,ΣZ⊗m), with m≥1, and let A be a deterministic, permutation-invariant size-indexed learning rule that outputs members of F and assume the maps (S,z)↦LAS(z) and (S,z)↦LAS∖i(z) for every i∈[m] are jointly measurable with respect to ΣZ⊗m⊗ΣZ. Assume also that the sample-dependent risk, empirical-risk, and leave-one-out maps in the conclusions are measurable. Assume 0≤LAT(z)≤M<∞ for every full or deleted sample T and every z∈Z, and assume A has uniform deletion stability with a finite nonincreasing sequence (βn)n≥1. Define Rloo(A,S)=m−1∑i=1mLAS∖i(Zi), using the size-m deletion output. Then

ES∼Pm[RP(AS)−R^S(AS)]≤2βmandES∼Pm[RP(AS)−Rloo(A,S)]≤βm.

Moreover, for every sample S and index i∈[m],

∣RP(AS)−RP(AS∖i)∣≤βm.

All three conclusions are choice-free.

Statement references

Bousquet and Elisseeff, Lemma 7 and proof, printed pp. 504–505, give the expected generalization and leave-one-out identities under the deletion convention. Full JMLR paper PDF

Mohri, Rostamizadeh, and Talwalkar, Theorem 14.2 and complete proof, printed pp. 334–336, give the corresponding replacement-stability argument. Full second-edition book PDF

Facts & Assumptions

Given: A supervised prediction problem with law P, an IID product sample S∼Pm, a measurable size-indexed learning rule, bounded measurable losses, and uniform deletion-stability coefficient βm.

[F1]

A supervised prediction problem fixes measurable instance and label spaces, a joint sample law, a class of measurable predictors, and a measurable loss. Supervised prediction problems

[F2]

Population risk is the integral of the predictor's loss against P, and empirical risk is the finite average of its sample losses. Population and empirical risk

[F3]

IID means mutually independent random elements with the same law; here the coordinate sample is the finite product sample with common law P. Identical distribution and IID families

[F4]

The finite product sigma-algebra is generated by coordinate rectangles; finite product measures have the rectangle formula and are unique, so permuting coordinates preserves the product law. The product sigma-algebra and its finite iterates For sigma-finite factors, the product measure exists, has the rectangle formula, is sigma-finite, and is unique

[F5]

For integrable measurable functions on a product, the product integral equals the iterated section integrals. Fubini's theorem for L^1 functions on a sigma-finite product

[F6]

A measurable function is in L1 when its absolute value has finite integral; the stated uniform bound on a probability product gives this integrability. Integrable real and complex functions, and their integrals

[F7]

The integral is linear on L1. The Lebesgue integral is linear on L1(μ)

[F8]

For h∈L1, ∣∫h dP∣≤∫∣h∣ dP. The modulus of an integral is bounded by the integral of the modulus

[F9]

Uniform deletion stability bounds the loss change between the full output and its size-m deleted output by βm, uniformly over samples, deleted indices, and test points. Uniform algorithmic stability under deletion

[A1]

The sample-dependent loss and risk maps are jointly measurable and all losses used here lie in [0,M].

Proof

1.1A1F1F2F3F4F5F6givenconstruct

Fix i∈[m] and extend the coordinate sample to (Zm+1,Pm+1), writing Zi′ for the last coordinate. By [A1] and [F6], (S,z′)↦LAS(z′) is integrable on this product space, and [F5] identifies ESRP(AS) with ES,Zi′LAS(Zi′). On each all-coordinate rectangle, the Pm+1 mass is the product of its coordinate masses. Transposing coordinates i and m+1 preserves that value; the generating and uniqueness statements in [F4] therefore show that the transposed product law is again Pm+1. If S(i) replaces Zi by Zi′, this gives ES,Zi′LAS(Zi′)=ES,Zi′LAS(i)(Zi).

1.2A1F2F7F8F9given

By [F2] and [F7], RP(AS)−RP(AS∖i)=∫Z(LAS(z)−LAS∖i(z)) P(dz). The integrand has absolute value at most βm by [F9], so [F8] gives ∣RP(AS)−RP(AS∖i)∣≤∫βm dP=βm.

1.3A1F3F4F5F6F7givenconstruct

For each i, the deleted output AS∖i depends only on the retained coordinates. Since Zi is independent of them and has law P, [F5] and [F6] give ESLAS∖i(Zi)=ESRP(AS∖i). Averaging over i therefore yields ES[RP(AS)−Rloo(A,S)]=m−1∑i=1mES[RP(AS)−RP(AS∖i)].

2.1F2F9step 1.1algebra

By [F2] and linearity, step 1.1 gives ES[RP(AS)−R^S(AS)]=m−1∑i=1mES,Zi′[LAS(i)(Zi)−LAS(Zi)]. The indexed retained samples S∖i and S(i)∖i coincide. Applying [F9] to each full output and that common deletion output, then the triangle inequality, bounds the absolute value of every summand by 2βm. Averaging gives the first expected bound.

3.1step 1.2step 1.3algebra∎

Each summand in the average in step 1.3 is at most βm by step 1.2, so its average is at most βm. This proves the leave-one-out bound.

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