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High-probability generalization from uniform stability

Statement

Assume the Axiom of Choice. 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 has a nonincreasing uniform deletion-stability sequence (βn)n≥1. Assume the maps (S,z)↦LAS(z) and (S,z)↦LAS∖i(z) for every i∈[m] are jointly measurable on the sample/test product space, and all sample-dependent risk, empirical-risk, and leave-one-out maps in the conclusions are measurable, and assume 0≤LAT(z)≤M<∞ for every full or deleted sample T and every z∈Z. Define Rloo(A,S)=m−1∑i=1mLAS∖i(Zi) using the size-m deletion output. For each δ∈(0,1), the following two bounds hold separately, each with probability at least 1−δ over S∼Pm:

RP(AS)≤R^S(AS)+2βm+(4mβm+M)log⁡(1/δ)2m,

RP(AS)≤Rloo(A,S)+βm+(4mβm+M)log⁡(1/δ)2m.

The displayed probability guarantees are separate and need not hold on the same event.

Statement references

The natural logarithm in the confidence terms is defined by The natural logarithm as the inverse of the exponential function.

Its positivity on arguments greater than 1 follows from Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm.

Bousquet and Elisseeff, Theorem 12 and full proof, printed pp. 507–509, give these two separate bounds under deletion stability. Full JMLR paper PDF

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

Facts & Assumptions

Given: AC, an IID product sample, a measurable supervised loss, a permutation-invariant rule with uniform deletion stability, jointly measurable sample-dependent test losses, measurable sample-dependent risks, and the common finite loss bound M.

[A1]

Assume AC. It is used through the bounded-differences supplier, whose conditional-expectation and martingale proof assumes AC. The Axiom of Choice

[A2]

The full and deleted sample-dependent test losses are jointly measurable in the sample and test observation. The sample-dependent risk, empirical-risk, and leave-one-out maps are measurable, and 0≤LAT(z)≤M.

[F1]

Population risk is the P-integral of the loss; empirical risk is its finite sample average. Population and empirical risk

[F2]

Uniform deletion stability bounds the pointwise loss change from a full output to its size-m deletion output by βm. Uniform algorithmic stability under deletion

[F3]

IID means mutually independent sample coordinates with common law P. Identical distribution and IID families

[F4]

The expected stability theorem gives E[RP(AS)−R^S(AS)]≤2βm, E[RP(AS)−Rloo(A,S)]≤βm, and the pointwise risk-change bound ∣RP(AS)−RP(AS∖i)∣≤βm. Expected generalization from uniform stability

[F5]

For x>0, log⁡x is the unique real y with exp⁡(y)=x. The natural logarithm as the inverse of the exponential function

[F6]

The natural logarithm is strictly increasing and log⁡1=0, so x>1 implies log⁡x>0. Order, continuity, range, and the product, quotient, and reciprocal laws for the natural logarithm

[F7]

Under AC, for independent coordinates and a product-measurable integrable function whose ith coordinate oscillation is at most ci, McDiarmid gives P{F−EF≥t}≤exp⁡(−2t2/∑ici2) for t>0 when ∑ici2>0. McDiarmid bounded-differences inequality

Proof

1.1A2F1F2F4givenalgebra

Fix an index k and let S′ be any sample that differs from S only at coordinate k. Their retained samples S∖k and S′∖k coincide. By [F4], ∣RP(AS)−RP(AS′)∣≤∣RP(AS)−RP(AS∖k)∣+∣RP(AS′∖k)−RP(AS′)∣≤2βm. For every j≠k, the two loss terms at the common test point Zj differ by at most 2βm, by [F2] through the same deleted output. The two kth loss terms each lie in [0,M], so their difference is at most M. Hence ∣R^S(AS)−R^S′(AS′)∣≤2(m−1)βm/m+M/m≤2βm+M/m. For Φ(S)=RP(AS)−R^S(AS) this yields the coordinate oscillation ∣Φ(S)−Φ(S′)∣≤4βm+M/m.

1.2A2F2F4givenconstructalgebra

Fix an index k and samples S,S′ differing only at coordinate k. The risk term in Ψ(S)=RP(AS)−Rloo(A,S) changes by at most 2βm by [F4]. In the leave-one-out average, the kth term uses the same deleted training sample for S and S′, and its test losses differ by at most M. For j≠k, the two deleted training samples S∖j and S′∖j have the same m−2 common observations and the two alternative kth observations. Form a size-m indexed sample T from those m−2 common observations and both alternatives. Up to permutation, each of the two deleted training samples is obtained by deleting one of the alternatives from T. Permutation invariance and [F2] therefore bound their loss change at the common test point Zj by 2βm. When m=1 there are no indices j≠k, so only the kth term occurs. Thus the leave-one-out average changes by at most 2(m−1)βm/m+M/m≤2βm+M/m, and ∣Ψ(S)−Ψ(S′)∣≤4βm+M/m.

2.1A1A2F3F4F5F6F7step 1.1givenalgebra

Put c=4βm+M/m. If c=0, then M=βm=0 and every loss and both gaps vanish. If c>0, then Φ is bounded, measurable, and integrable by [A2]. Since 0<δ<1, [F5]–[F6] give log⁡(1/δ)>0, so t=cmlog⁡(1/δ)/2>0. The product law Pm makes the coordinates independent by [F3], and [F7] with all m coordinate bounds equal to c gives P{Φ−EΦ≥t}≤exp⁡(−2t2/(mc2))=δ. By [F4], outside this event Φ≤2βm+t=2βm+(4mβm+M)log⁡(1/δ)/(2m). Together with the c=0 case this proves the first probability bound.

3.1A1A2F3F4F5F6F7step 1.2givenalgebra∎

With c=4βm+M/m, the case c=0 again has zero losses and zero gaps. If c>0, [A2] makes Ψ bounded, measurable, and integrable. Since 0<δ<1, [F5]–[F6] give log⁡(1/δ)>0. The product law Pm makes the coordinates independent by [F3], and [F7] with coordinate bounds c gives P{Ψ−EΨ≥cmlog⁡(1/δ)/2}≤δ. By [F4], EΨ≤βm, so outside this event Ψ≤βm+(4mβm+M)log⁡(1/δ)/(2m). This proves the second probability bound, separately from the first.

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