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An extra square-root sample-size factor destroys standard-error studentization

Statement refuted

If Vm,nSE is the estimated standard error in Studentized two-sample U-statistic and permutation distribution and Sm,n=Um,n/Vm,nSE, then the extra scaling m Sm,n still has a standard-normal limit.

Facts & Assumptions

Given: AC, the Rademacher law, h(x,y)=x−y, proportional sample sizes, and the sample-variance studentizer.

[F1]

Two-sample U-statistic parameter: for r=1, θ(P,Q;h)=EPX−EQY.

[F2]

Identical distribution and IID families: the two samples are independent IID Rademacher samples.

[F3]

Moments, variance, and covariance on a probability space: variance is the second centered moment.

[F4]

Chebyshev's inequality on a finite probability space: sample averages of bounded IID variables converge in probability to their expectations.

[F5]

Studentized two-sample U-statistic and permutation distribution: standard-error studentization sets Sm,n=Um,n/Vm,nSE.

[F6]

Studentized permutation validity under mixed-assignment centering: when its stated hypotheses hold, both Sm,n⇒N(0,1) and its conditional permutation CDF converge uniformly in probability to Φ.

[A1]

AC is assumed by the asymptotic permutation limit result [F6]. The Axiom of Choice

Counterexample

Let P=Q be the Rademacher law on {−1,1}, take r=1 and h(x,y)=x−y, and let m/(m+n)→p∈(0,1). Use the two usual sample variances σ^k2=1k−1∑i=1k(Wi−Wˉk)2,Wˉk=1k∑i=1kWi, in Vm,nSE.

Given: P=Q Rademacher, h(x,y)=x−y, and the usual sample variances. Verify the standard-normal limits and refute the extra-scaled claim.

1.1F1F2F3algebra

The two population means are zero and the two variances are one, so θ(P,Q;h)=0 by [F1]. The kernel is antisymmetric, and both P/Q assignments have mean zero. For each assignment, h2 equals 0 on the two equal-sign pairs and 4 on the two opposite-sign pairs, each of probability 1/4, so E[h2]=2. The first projections are gX(x)=E[x−Y]−θ=x and gY(y)=E[X−y]−θ=−y; each has variance 1.

1.2A1F2F3F4F5F6givenalgebra

Under AC [A1], the sample variances are measurable finite-sample statistics. Here Wi2=1 exactly and, by [F4], the sample mean Wˉ converges in probability to 0; the identity Var⁡^(W)=kk−1(1−Wˉ2) then gives consistency to 1. The pooled mixture is the same Rademacher law, so this consistency holds in both permuted groups and the positive pooled projection variance is also 1. Thus [F6] applies and gives the standard-normal limits for Sm,n itself and its conditional permutation distribution.

2.1F6step 1.2algebra

For every fixed M>0 and every ε>0, eventually M/m<ε, hence P(∣m Sm,n∣≤M)≤P(∣Sm,n∣≤ε). By the true-sampling convergence in step 1.2, the limsup of the right side is at most Φ(ε)−Φ(−ε). Letting ε↓0 proves P(∣m Sm,n∣≤M)→0; thus ∣m Sm,n∣→∞ in probability.

3.1A1F6step 1.2algebra∎

Under AC [A1], let FNπ be the conditional permutation CDF of Sm,nπ. For fixed ε>0, eventually M/m<ε, so Pπ(∣m Sm,nπ∣≤M∣Z)≤FNπ(ε)−FNπ(−ε−), where FNπ(−ε−) denotes the left limit of the conditional CDF. By [F6], the right side converges in probability to Φ(ε)−Φ(−ε). Letting ε↓0 shows that the conditional probability of any fixed bounded interval tends to zero in probability. The standard-normal pivot is Sm,n; the extra m factor is erroneous when the denominator is already a standard error.

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