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Independent-subject terminal influence limits — Examples

1 · Prerequisites

2 · Summary

The first example verifies an exact Bernoulli sample-mean expansion, and the second calculates the covariance for two fixed-weight independent groups. The final example uses a shrinking risk set whose denominator is empty with probability one half, so its root-n error is not tight. These examples distinguish applying a terminal limit theorem from proving the estimator-specific expansion it requires.

3 · Logical flowchart

4 · Definitions, theorems and proofs

None yet.

5 · Examples, counterexamples and false statements

ExampleConstruction: AI-generatedVerification: AI-generatedprecheck passjudge pass (gpt-6-luna)Open item page →

Bernoulli sample mean as an exact terminal influence expansion

Example

Assume AC. Let B1,B2,… be IID Bernoulli(p) variables, where 0≤p≤1, and estimate θ=p by θ^n=Bˉn=n−1∑i=1nBi. Take the subject influence function ϕ(b)=b−p.

Facts & Assumptions

Given: The Bernoulli sample and estimator in the Example. Verify its influence expansion and terminal limit.

[A1]

The observations are IID Bernoulli(p), with P(Bi=1)=p and P(Bi=0)=1−p.

[A2]
[F1]

Identical distribution and IID families: IID means a common law and mutual independence of the whole family.

[F2]

Moments, variance, and covariance on a probability space: For a square-integrable real random variable X, Var⁡(X)=E[(X−EX)2].

[F3]

IID terminal influence limit from a separately established expansion: A separately supplied centered, finite-variance IID influence expansion has its covariance normal limit under AC.

Verification

1.1A1F1F2algebra

For every Borel set C⊆R, {Bi−p∈C}={Bi∈C+p}, where C+p is Borel. The common law of the Bi therefore gives a common law for Bi−p, and applying the finite independence identities in [F1] to these preimage events proves that the shifted observations are IID. The variable ϕ(Bi)=Bi−p has mean p−p=0 and variance p(1−p) by [A1] and [F2]. The estimator identity Bˉn−p=n−1∑i(Bi−p) gives the required expansion exactly, with zero remainder.

2.1A2F3step 1.1∎

Apply [F3] with d=1 and Σ=p(1−p) to obtain n(Bˉn−p)⇒N(0,p(1−p)). At p=0 or p=1, Bˉn=p almost surely, so the limit is the point mass N(0,0) covered by the theorem.

ExampleConstruction: AI-generatedVerification: AI-generatedprecheck passjudge pass (gpt-6-luna)Open item page →

Weighted means from two independent subject groups

Example

Assume AC. Let X1i be IID Bernoulli(p1) and X2i IID Bernoulli(p2), with the two subject groups independent. Let deterministic sample sizes satisfy n1+n2=n, n1/n→ρ∈(0,1), and n2/n→1−ρ. For g∈{1,2} define Xˉg=ng−1∑i=1ngXgi. For the fixed target θ=ρp1+(1−ρ)p2, use the fixed-allocation-weight estimator θ^n=ρXˉ1+(1−ρ)Xˉ2.

Facts & Assumptions

Given: The two independent Bernoulli groups and estimator in the Example. Find its exact group influence expansion and limit.

[A1]

In group g, P(Xgi=1)=pg and P(Xgi=0)=1−pg; the observations are IID within groups and the two groups are independent.

[A2]
[F1]

Identical distribution and IID families: IID means a common law and mutual independence of the whole family.

[F2]

Independent families of event classes: For every finite selection of distinct independent indices and one event from each selected class, the probability of their intersection is the product of their probabilities.

[F3]

Moments, variance, and covariance on a probability space: For a square-integrable real random variable X, Var⁡(X)=E[(X−EX)2].

[F4]

Independent-group terminal influence limit: For a fixed finite family of independent IID groups, the covariance of the terminal normal limit is ∑gρg−1Σg, provided the stated group expansion and hypotheses hold.

Verification

1.1A1F1F2F3F4algebra

Put ϕ1(x)=ρ(x−p1) and ϕ2(x)=(1−ρ)(x−p2). The measurable maps preserve within-group identical laws and the finite event-product identities for independence across groups, so their sampled values meet [F1] and [F2], as required by [F4]. By [A1] and [F3], they are centered, square-integrable group contributions with variances Σ1=ρ2p1(1−p1) and Σ2=(1−ρ)2p2(1−p2). The identity n(θ^n−θ)=n/n1 n1−1/2∑i=1n1ϕ1(X1i)+n/n2 n2−1/2∑i=1n2ϕ2(X2i) is exact, so its remainder is zero.

2.1A2F4step 1.1algebra∎

The covariance in [F4] is ρ−1Σ1+(1−ρ)−1Σ2=ρp1(1−p1)+(1−ρ)p2(1−p2). Thus n(θ^n−θ) converges to the centered normal with this variance. The formula includes pg∈{0,1}, where the corresponding group variance is zero.

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An empty risk set prevents root-n tightness

Example

Assume AC. Let T1,T2,… be IID uniform on [0,1] event times and set Xi=1 for every subject. For each n≥2, use the moving terminal time tn=2−1/n and risk indicators Yni=1{Ti≥tn}. Write Kn=∑i=1nYni. The target mean among subjects at risk is the fixed value θ=1. Define the risk-set ratio

θ^n={∑i=1nXiYniKn,Kn>0,0,Kn=0.

The convention makes the estimator defined even when the risk set is empty.

Facts & Assumptions

Given: The IID uniform event times and ratio estimator in the Example. Show that its root-n error is not tight, so the terminal influence theorem cannot be applied without a separate expansion.

[A1]

The event times are IID, their common law satisfies P(Ti<t)=t for 0≤t≤1, and subjects are mutually independent.

[A2]
[F1]

Identical distribution and IID families: IID means a common law and mutual independence of the whole family.

[F2]

Independent families of event classes: For finite selections of distinct independent indices, the probability of an intersection is the product of the event probabilities.

[F3]

Stochastic order and convergence notation for finite-dimensional random vectors: Zn=OP(1) requires that its tail probabilities can be made uniformly small in the limit by choosing a sufficiently large bound.

[F4]

Finite-dimensional stochastic-order calculus and tightness: Any finite-dimensional sequence converging in distribution is bounded in probability.

[F5]

IID terminal influence limit from a separately established expansion: If this estimator had a centered finite-variance IID influence expansion, its root-n error would converge in distribution under AC.

Verification

1.1A1F1F2algebra

For each n≥2, [A1] gives P(Yni=0)=tn=2−1/n. The events {Yni=0} are independent by the finite event-product identity in [F2], hence P(Kn=0)=(2−1/n)n=1/2. Since Xi=1, the ratio is 1 when Kn>0 and is 0 otherwise; therefore n(θ^n−1) is 0 when Kn>0 and −n when Kn=0.

2.1A2F3F4F5step 1.1∎

For every fixed M>0 and every integer n>M2, P(∣n(θ^n−1)∣>M)=P(Kn=0)=1/2. Taking η=1/4 in [F3] shows that the root-n errors are not OP(1). If they converged in distribution, [F4] would make them OP(1), a contradiction. Under [A2], the expansion required by [F5] would imply a weak limit, so it cannot hold for this estimator. The example shows why a consumer must control its own random denominator before claiming a terminal influence limit.