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Terminal normal limit for weighted logrank scores

Statement

Assume the Axiom of Choice. For g∈{0,1}, let (Tgi,Cgi)i≥1 be IID within group, with Tgi,Cgi∈[0,∞) almost surely and the two group families independent. Assume conditional independent censoring Tgi⊥Cgi∣g, and a common absolutely continuous event-time law with hazard density λ on [0,τ], where Λ(τ)=∫0τλ(t) dt<∞. Censoring distributions may differ between groups. Use the horizon-truncated record Ugi:=min⁡(Tgi,Cgi,τ) and δgi:=1{Tgi≤Cgi, Tgi≤τ}, which lies in [0,τ]×{0,1} as required by Two-group logrank and weighted logrank scores.

For deterministic positive sample sizes ng along a sequence with n=n0+n1→∞ and n1/n→ρ∈(0,1), set ρ1=ρ, ρ0=1−ρ, yg(t)=ρgP(Ug1≥t), y(t)=y0(t)+y1(t), and zˉ(t)=y1(t)/y(t). Assume inf⁡t≤τy(t)>0. For each subject define Ngi(t)=1{Ugi≤t,δgi=1} and Ygi(t)=1{Ugi≥t}. Under one common filtration, assume Ygi is predictable and, separately, that Agi(t)=∫0tYgi(s)λ(s) ds,Mgi=Ngi−Agi satisfies the finite-horizon square-integrable martingale contract of Simple counting-process intensity and compensator contract. Assume no two subjects have simultaneous event jumps. Conditional independent censoring and the displayed hazard are scientific model premises; they are not used as substitutes for the explicit martingale contract.

Use Un,wn and V^n,wn as defined in Two-group logrank and weighted logrank scores. Let wn be a common predictable weight sequence with ∣wn∣≤C and a measurable supremum satisfying sup⁡t≤τ∣wn(t)−w(t)∣→P0 for a deterministic bounded Borel predictable weight w. Define σw2:=∫0τw(t)2y0(t)y1(t)y0(t)+y1(t)λ(t) dt. Then Un,wnn⇒N(0,σw2). If σw2>0, define the standardized statistic as Un,wn/V^n,wn when the denominator is positive and assign any fixed finite value on the event that it is zero. Every such extension converges to N(0,1). If σw2=0, only the degenerate score limit is asserted. For a fixed deterministic weight, take wn=w. This is a terminal finite-dimensional limit; it asserts no process central limit theorem.

Facts & Assumptions

Given: Assume AC; two independent IID groups of subject records, deterministic allocation fractions tending to ρg∈(0,1), equal continuous event hazard, group-conditional independent censoring, the explicit counting-process contracts and no-simultaneous-jump condition, positive pooled risk support, finite Λ(τ), and the bounded predictable weights in the Statement.

[A1]

Assume the Axiom of Choice (AC), inherited from the counting-process integral and independent-groups terminal-limit suppliers. (The Axiom of Choice)

[A2]

Since each Ugi is the same measurable function of its within-group IID pair (Tgi,Cgi), the observed times are IID within each group. The deterministic allocation limits and positive pooled-risk bound in the Statement therefore meet the hypotheses of Uniform convergence of the risk-set treatment fraction, which supplies the uniform risk-fraction limits required by [F4] and [F5].

[F1]

Identical distribution and IID families: within each group the subject records share one law and are mutually independent.

[F2]

Independent families of event classes: independence of the two subject families means every finite choice of events from distinct subjects has product probability.

[F3]

Two-group logrank and weighted logrank scores: Un,wn and V^n,wn are the weighted risk-set score and its plug-in variance sum.

[F4]

Weighted logrank score expansion under a common-hazard model: under its stated martingale, risk-fraction, no-simultaneous-jump, and weight hypotheses, Un,wn/n=n−1/2∑g=01∑i=1ngϕg(Ugi,δgi)+oP(1), where ϕg=∫0τw(t)(g−zˉ(t)) dMgi(t).

[F5]

Consistency of the weighted logrank variance estimate: V^n,wn/n→Pσw2 under its stated continuous-event and plug-in variance hypotheses.

[F6]

Independent-group terminal influence limit: for a fixed finite number of independent IID groups, centered square-integrable measurable group influences and a separately established terminal expansion imply a Gaussian limit with covariance ∑gρg−1Σg.

[F7]

Simple counting-process intensity and compensator contract: Mgi=Ngi−Agi is a square-integrable martingale under the explicit model contract, and Agi=∫Ygiλ.

[F8]

L² counting-process integral when the limit exists: predictable integrands of finite compensator energy have the stated L2 counting-process integral when the approximating integrals converge.

[F9]

L² extension of the predictable counting-process integral: the integral is a square-integrable martingale and E(∫H dM)2=E∫H2 dA for finite-energy predictable H.

[F10]

Covariance matrix of a finite-dimensional random vector: for a centered square-integrable scalar, its covariance is its second moment.

[F11]

Tonelli's theorem for nonnegative measurable functions on a sigma-finite product: a nonnegative measurable integrand may be integrated in either order on the product space.

[F12]

Expectations factor over finite products of independent random variables: expectations of products of finitely many independent integrable real random variables factor into the product of their expectations.

[F13]

Variance of a finite sum as the sum of all variances and covariances: the variance of a finite sum is the sum of its variances and pairwise covariance terms.

[F14]

Markov's inequality for random variables: for X≥0 and a>0, P(X≥a)≤E[X]/a.

[F15]

Dominated convergence: pointwise convergence under one integrable majorant permits passage to the limit in the integral.

[F16]

Stochastic order and convergence notation for finite-dimensional random vectors: Xn=OP(1) means its tails are bounded in probability, and Xn=oP(1) means convergence in probability to zero.

[F17]

Standard normal and normal laws: N(0,v) is the law of vZ for Z∼N(0,1), including v=0.

[F18]

Vector Slutsky theorem: weak convergence is preserved after adjoining a quantity converging in probability to a constant and applying a continuous map.

Proof

1.1A1F1F2F7F8F9given

Put hg(t)=w(t)(g−zˉ(t)). For a subject record (u,δ), define the real map ϕg(u,δ)=1{δ=1, u≤τ}hg(u)−∫0u∧τhg(t)λ(t) dt. The deterministic integrand is bounded and Borel, so its indefinite integral is Borel and this map is measurable. From the definitions of Ngi, Ygi and Agi, this is exactly ∫0τhg(t) dMgi(t). Its energy is at most ∥hg∥∞2Λ(τ); [F8] and [F9] therefore make it square-integrable and centered. It is the same measurable function of each subject record within group g, so [F1] makes the transformed records IID within group and [F2] makes the two groups independent.

2.1F9F11givenstep 1.1algebra

By [F9] and [F11], E[ϕg2]=∫0τw(t)2(g−zˉ(t))2P(Ug1≥t)λ(t) dt. Multiplying by ρg and summing over g=0,1 gives ∑g=01ρgE[ϕg2]=∫0τw(t)2{y1(t)(1−zˉ(t))2+y0(t)zˉ(t)2}λ(t) dt=σw2, where the last equality follows by substituting zˉ=y1/(y0+y1). The integral is finite since w is bounded, 0≤yg≤1, and Λ(τ)<∞.

2.2F1F12F13F14F15F16step 1.1

Fix ϵ>0. Since ng→∞, the integrand ϕg21{∣ϕg∣>ϵng} tends pointwise to zero and is bounded by the integrable ϕg2. [F15] gives ngP(∣ϕg∣>ϵng)≤ϵ−2E[ϕg21{∣ϕg∣>ϵng}]⟶0. Thus each group satisfies the finite-variance no-large-jump condition. Also, for Sg,n=ng−1/2∑i=1ngϕg(Ugi,δgi), [F1] and [F12] make the cross covariances zero, and [F13] gives E[Sg,n2]=E[ϕg2]. Applying [F14] to Sg,n2 yields P(∣Sg,n∣>M)≤E[ϕg2]/M2; hence Sg,n=OP(1) by [F16].

3.1A2F3F4F6step 2.2algebra

By [A2], the risk-fraction hypotheses used by [F4] hold. Let θ^n=Un,wn/n and set θ=0. By [F4], n(θ^n−θ)=n−1/2∑g=01∑i=1ngϕg(Ugi,δgi)+oP(1). Set ψg=ρgϕg. The group-sum term in [F6] is Sn∗=∑g=01nng 1ng∑i=1ngψg(Ugi,δgi). Its difference from n−1/2∑g,iϕg(Ugi,δgi) equals ∑g=01nng(ρg−ngn)Sg,n=oP(1), because Sg,n=OP(1) by step 2.2, ng/n→ρg>0, and there are only two groups. Thus the expansion required by [F6] is established with centered group influences ψg.

4.1A1F6F10step 1.1step 2.1step 3.1

Each ψg is centered and square-integrable by step 1.1, and Var⁡(ψg)=ρg2E[ϕg2] by [F10]. The covariance in [F6] is therefore ∑g=01ρg−1Var⁡(ψg)=∑g=01ρgE[ϕg2]=σw2 by step 2.1. Applying [F6] to the proved expansion in step 3.1 yields Un,wn/n⇒N(0,σw2).

5.1A2F5F17F18step 4.1∎

Suppose σw2>0 and put Vn=V^n,wn/n. By [A2] and [F5], Vn→Pσw2, so P(Vn>σw2/2)→1. The function (x,v)↦x/max⁡(v,σw2/2) is continuous everywhere. Apply [F18] and this map to the score limit and Vn; the result converges to Z/σw for Z∼N(0,σw2), which has law N(0,1) by [F17]. On {Vn>σw2/2} this map equals the stated standardized statistic. Since the complement has probability tending to zero, changing the statistic there has no effect on weak convergence, including any fixed finite extension on the zero-denominator event. When σw2=0, step 4.1 gives the degenerate score law and no standardized conclusion.

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