CLT under Local Dependence: When Observations Are Connected

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When observations share random inputs, the variance of their sum includes covariances between them. We study when dividing the centered sum by its standard deviation gives an approximately standard normal distribution. A dependency graph specifies which groups of observations must be independent. Under this condition, a bound from Stein's method gives sufficient conditions for a central limit theorem (CLT). We then compare it with a bound that directly controls the difference between distribution functions.

The starting result is Ross’s Theorem 3.5. We give the probabilistic argument in full under an explicit dependency-graph assumption. The later Chen–Shao theorem is stated with its conditions and applied to the same example; its longer concentration-inequality proof is not reproduced here.

1. The sum and its dependency graph

Let (X1,…,Xn)∼Pn(X_1,\ldots,X_n)\sim P_n be a real random vector with EPnXi=0\mathbb E_{P_n}X_i=0 and finite fourth moments. The observations may be dependent, so PnP_n need not be a product of their marginal distributions. Throughout the graph proof, unmarked E\mathbb E, Var⁡\operatorname{Var}, Cov⁡\operatorname{Cov}, and P\mathbb P use this joint distribution or its relevant marginal. The graph is fixed; we do not average over randomly generated graphs. Define

S=∑i=1nXi,σ2=Var⁡(S)>0,W=Sσ.S=\sum_{i=1}^n X_i,\qquad \sigma^2=\operatorname{Var}(S)>0,\qquad W=\frac{S}{\sigma}.

A deterministic undirected graph GG on {1,…,n}\{1,\ldots,n\} is a dependency graph if, for any disjoint vertex sets I,JI,J with no edge between them,

(Xi)i∈I ⊥ ⁣ ⁣ ⁣⊥ (Xj)j∈J.(X_i)_{i\in I}\ \perp\!\!\!\perp\ (X_j)_{j\in J}.

This is independence of two random vectors, not merely pairwise independence. For example, if U,VU,V are independent random signs, then U,V,UVU,V,UV are pairwise independent, but

U⋅V⋅(UV)=1almost surely.U\cdot V\cdot(UV)=1\quad\text{almost surely}.

The empty graph is therefore not a dependency graph for this triple.

Write

Ni={i}∪{j:{i,j} is an edge of G},D=max⁡i∣Ni∣.N_i=\{i\}\cup\{j:\{i,j\}\text{ is an edge of }G\},\qquad D=\max_i|N_i|.

Thus DD counts the vertex itself; it is one plus the maximum graph degree. Because the graph is undirected, j∈Nij\in N_i if and only if i∈Nji\in N_j. Nonneighbors are independent, so

σ2=∑i=1n∑j=1nE[XiXj]=∑i=1n∑j∈NiE[XiXj].\begin{aligned} \sigma^2 &=\sum_{i=1}^n\sum_{j=1}^n\mathbb E[X_iX_j]\\ &=\sum_{i=1}^n\sum_{j\in N_i}\mathbb E[X_iX_j]. \end{aligned}

Thus normalization must include covariances between distinct neighboring observations, even when each observation depends on only a small neighborhood.

Constructing the graph from independent inputs

Suppose {εv:v∈V}\{\varepsilon_v:v\in\mathcal V\} are mutually independent, and each observation has the form

Xi=gi((εv)v∈Bi),Bi⊆V.X_i=g_i\bigl((\varepsilon_v)_{v\in B_i}\bigr),\qquad B_i\subseteq\mathcal V.

Joining ii and jj whenever Bi∩Bj≠∅B_i\cap B_j\ne\varnothing gives a dependency graph: two groups with no connecting edge are functions of disjoint sets of independent inputs.

For example, when BiB_i is the closed one-hop neighborhood of ii in an underlying network, the dependency graph may require edges between vertices at network distance two. If the underlying maximum degree is dd, then

D≤1+d+d(d−1)=1+d2.D\le1+d+d(d-1)=1+d^2.

This construction requires the independent-input representation. A social or spatial network by itself does not establish the required independence.

2. Why the normal distribution leads to Stein’s equation

Let Z∼N(0,1)Z\sim N(0,1), with density φ(x)=(2π)−1/2e−x2/2\varphi(x)=(2\pi)^{-1/2}e^{-x^2/2} and distribution function Φ\Phi. Expressions Eh(Z)\mathbb E h(Z) integrate only Z∼N(0,1)Z\sim N(0,1); expressions Eh(W)\mathbb E h(W) integrate W=∑iXi/σW=\sum_iX_i/\sigma under PnP_n. The comparison requires no joint coupling of WW and ZZ. Since φ′(x)=−xφ(x)\varphi'(x)=-x\varphi(x), integration by parts gives, for a bounded continuously differentiable ff with bounded derivative,

Ef′(Z)=∫Rf′(x)φ(x) dx=[f(x)φ(x)]−∞∞−∫Rf(x)φ′(x) dx=E[Zf(Z)].\begin{aligned} \mathbb E f'(Z) &=\int_{\mathbb R}f'(x)\varphi(x)\,dx\\ &=[f(x)\varphi(x)]_{-\infty}^{\infty} -\int_{\mathbb R}f(x)\varphi'(x)\,dx\\ &=\mathbb E[Zf(Z)]. \end{aligned}

To compare WW with ZZ, take a test function h:R→Rh:\mathbb R\to\mathbb R and solve

fh′(w)−wfh(w)=h(w)−Eh(Z).f_h'(w)-wf_h(w)=h(w)-\mathbb E h(Z).

Multiplication by e−w2/2e^{-w^2/2} yields an integrating factor:

ddw{e−w2/2fh(w)}=e−w2/2{h(w)−Eh(Z)},fh(w)=ew2/2∫−∞we−t2/2{h(t)−Eh(Z)} dt.\begin{aligned} \frac{d}{dw}\{e^{-w^2/2}f_h(w)\} &=e^{-w^2/2}\{h(w)-\mathbb E h(Z)\},\\ f_h(w) &=e^{w^2/2}\int_{-\infty}^w e^{-t^2/2}\{h(t)-\mathbb E h(Z)\}\,dt. \end{aligned}

Consequently,

Eh(W)−Eh(Z)=E{fh′(W)−Wfh(W)}.\mathbb E h(W)-\mathbb E h(Z) =\mathbb E\{f_h'(W)-Wf_h(W)\}.

We first use the Wasserstein distance

dW(W,Z)=sup⁡Lip⁡(h)≤1∣Eh(W)−Eh(Z)∣,d_{\mathrm W}(W,Z) =\sup_{\operatorname{Lip}(h)\le1} \left|\mathbb E h(W)-\mathbb E h(Z)\right|,

where Lip⁡(h)≤1\operatorname{Lip}(h)\le1 means ∣h(x)−h(y)∣≤∣x−y∣|h(x)-h(y)|\le|x-y| for every x,yx,y. These expectations exist because WW and ZZ have finite first moments.

To use this distance, we need bounds on the solution of Stein's equation. The following Stein-factor lemma supplies them: for such hh, the bounded solution satisfies

∥fh′∥∞≤2π,Lip⁡(fh′)≤2.\|f_h'\|_\infty\le\sqrt{\frac2\pi},\qquad \operatorname{Lip}(f_h')\le2.

For smooth hh, the second inequality can be written ∥fh′′∥∞≤2\|f_h''\|_\infty\le2; the Lipschitz formulation also covers nonsmooth hh. We use these bounds from Ross, Lemma 2.5. They control the Taylor remainder below, leaving us to bound moments of local sums and count which products can be dependent.

3. A dependency-graph normal-approximation theorem

Theorem 1. Under the centering, fourth-moment, positive-variance, and dependency-graph assumptions above,

dW(W,Z)≤D2σ3∑i=1nE∣Xi∣3+26π D3/2σ2(∑i=1nEXi4)1/2.\boxed{ \begin{aligned} d_{\mathrm W}(W,Z) &\le \frac{D^2}{\sigma^3}\sum_{i=1}^n\mathbb E|X_i|^3\\ &\quad+\sqrt{\frac{26}{\pi}}\, \frac{D^{3/2}}{\sigma^2} \left(\sum_{i=1}^n\mathbb E X_i^4\right)^{1/2}. \end{aligned}}

This is the bound in Ross, Theorem 3.5, used here with the full graph-independence condition. The proof separates two errors: a Taylor remainder and the fluctuation of a local quadratic sum. No identical-distribution assumption is needed.

Removing the neighborhood creates independence

First, define

ξi=Xiσ,ηi=∑j∈Niξj,Wi=W−ηi=∑j∉Niξj.\xi_i=\frac{X_i}{\sigma},\qquad \eta_i=\sum_{j\in N_i}\xi_j,\qquad W_i=W-\eta_i=\sum_{j\notin N_i}\xi_j.

The graph condition gives ξi⊥ ⁣ ⁣ ⁣⊥Wi\xi_i\perp\!\!\!\perp W_i. Centering then implies

E[ξifh(Wi)]=Eξi Efh(Wi)=0,\mathbb E[\xi_i f_h(W_i)] =\mathbb E\xi_i\,\mathbb E f_h(W_i)=0,

and hence

E[Wfh(W)]=∑iE[ξi{fh(W)−fh(Wi)}].\mathbb E[Wf_h(W)] =\sum_i\mathbb E\left[\xi_i\{f_h(W)-f_h(W_i)\}\right].

Next, expand around WW. The fundamental theorem of calculus and the Lipschitz bound on fh′f_h' give

ri=fh(W)−fh(W−ηi)−ηifh′(W)=∫0ηi{fh′(W−t)−fh′(W)} dt,∣ri∣≤∫0∣ηi∣2t dt=ηi2.\begin{aligned} r_i &=f_h(W)-f_h(W-\eta_i)-\eta_i f_h'(W)\\ &=\int_0^{\eta_i}\{f_h'(W-t)-f_h'(W)\}\,dt,\\ |r_i|&\le\int_0^{|\eta_i|}2t\,dt=\eta_i^2. \end{aligned}

Define the normalized local quadratic sum

V=∑iξiηi=1σ2∑i∑j∈NiXiXj.V=\sum_i\xi_i\eta_i =\frac{1}{\sigma^2}\sum_i\sum_{j\in N_i}X_iX_j.

The variance identity in Section 1 gives EV=1\mathbb E V=1. Substituting the expansion into the Stein discrepancy therefore yields

E{fh′(W)−Wfh(W)}=E{fh′(W)(1−V)}−∑iE(ξiri),∣E{fh′(W)−Wfh(W)}∣≤2π E∣1−V∣+∑iE(∣ξi∣ηi2)≤2πVar⁡(V)+∑iE(∣ξi∣ηi2).\begin{aligned} \mathbb E\{f_h'(W)-Wf_h(W)\} &=\mathbb E\{f_h'(W)(1-V)\} -\sum_i\mathbb E(\xi_i r_i),\\ \left|\mathbb E\{f_h'(W)-Wf_h(W)\}\right| &\le\sqrt{\frac2\pi}\,\mathbb E|1-V| +\sum_i\mathbb E(|\xi_i|\eta_i^2)\\ &\le\sqrt{\frac2\pi}\sqrt{\operatorname{Var}(V)} +\sum_i\mathbb E(|\xi_i|\eta_i^2). \end{aligned}

The final inequality is Cauchy–Schwarz, using E(1−V)=0\mathbb E(1-V)=0. Taking the supremum over hh proves the intermediate bound

dW(W,Z)≤2πVar⁡(V)+∑iE(∣ξi∣ηi2).\boxed{ d_{\mathrm W}(W,Z) \le\sqrt{\frac2\pi}\sqrt{\operatorname{Var}(V)} +\sum_i\mathbb E(|\xi_i|\eta_i^2). }

This intermediate bound retains the individual neighborhoods and covariances. When they are known, evaluating it directly can avoid replacing every local contribution by a worst-case maximum-degree bound.

Bounding the Taylor remainder

Expanding the neighborhood square and applying the triangle inequality gives

∑iE(∣ξi∣ηi2)≤1σ3∑i∑j,k∈NiE∣XiXjXk∣.\sum_i\mathbb E(|\xi_i|\eta_i^2) \le\frac1{\sigma^3}\sum_i\sum_{j,k\in N_i} \mathbb E|X_iX_jX_k|.

By the arithmetic–geometric mean inequality,

∣XiXjXk∣≤∣Xi∣3+∣Xj∣3+∣Xk∣33.|X_iX_jX_k| \le\frac{|X_i|^3+|X_j|^3+|X_k|^3}{3}.

To count each contribution, write ai=E∣Xi∣3a_i=\mathbb E|X_i|^3. There are at most D2D^2 choices of (j,k)(j,k) for a fixed ii, while symmetry of the neighborhoods gives

∑i∑j,k∈Niai≤D2∑iai,∑i∑j,k∈Niaj=∑jaj∑i:j∈Ni∣Ni∣≤D2∑jaj.\begin{aligned} \sum_i\sum_{j,k\in N_i}a_i&\le D^2\sum_i a_i,\\ \sum_i\sum_{j,k\in N_i}a_j &=\sum_j a_j\sum_{i:j\in N_i}|N_i| \le D^2\sum_j a_j. \end{aligned}

The same count applies to aka_k. It follows that

∑iE(∣ξi∣ηi2)≤D2σ3∑iE∣Xi∣3.\sum_i\mathbb E(|\xi_i|\eta_i^2) \le\frac{D^2}{\sigma^3}\sum_i\mathbb E|X_i|^3.

Bounding the local quadratic fluctuation

For the other term, put

T=∑i∑j∈NiXiXj,V=T/σ2.T=\sum_i\sum_{j\in N_i}X_iX_j, \qquad V=T/\sigma^2.

Bilinearity of covariance gives

Var⁡(T)=∑i∑j∈Ni∑k∑ℓ∈NkCov⁡(XiXj,XkXℓ).\operatorname{Var}(T) =\sum_i\sum_{j\in N_i}\sum_k\sum_{\ell\in N_k} \operatorname{Cov}(X_iX_j,X_kX_\ell).

If {i,j}\{i,j\} and {k,ℓ}\{k,\ell\} are disjoint and no graph edge connects them, the corresponding products are independent and their covariance vanishes. Thus a potentially nonzero term must satisfy

k∈Ni∪Njorℓ∈Ni∪Nj.k\in N_i\cup N_j \quad\text{or}\quad \ell\in N_i\cup N_j.

Let Q\mathcal Q be the set of ordered quadruples satisfying j∈Nij\in N_i, ℓ∈Nk\ell\in N_k, and this last condition. For each fixed (i,j)(i,j), the first alternative allows at most 2D⋅D2D\cdot D pairs (k,ℓ)(k,\ell). The second allows at most another 2D⋅D2D\cdot D, since ℓ∈Nk\ell\in N_k is equivalent to k∈Nℓk\in N_\ell. Consequently,

#{(j,k,ℓ):(i,j,k,ℓ)∈Q}≤D⋅4D2=4D3.\#\{(j,k,\ell):(i,j,k,\ell)\in\mathcal Q\} \le D\cdot4D^2=4D^3.

The same bound holds with any one of the four positions fixed: Q\mathcal Q is invariant under interchanging the members of either pair and under interchanging the two pairs.

Write mi=EXi4m_i=\mathbb E X_i^4. Cauchy–Schwarz, followed by Hölder and the arithmetic–geometric mean inequality, gives

∣Cov⁡(XiXj,XkXℓ)∣≤Var⁡(XiXj)Var⁡(XkXℓ)≤E(Xi2Xj2)E(Xk2Xℓ2)≤(mimjmkmℓ)1/4≤14(mi+mj+mk+mℓ).\begin{aligned} |\operatorname{Cov}(X_iX_j,X_kX_\ell)| &\le\sqrt{\operatorname{Var}(X_iX_j) \operatorname{Var}(X_kX_\ell)}\\ &\le\sqrt{\mathbb E(X_i^2X_j^2) \mathbb E(X_k^2X_\ell^2)}\\ &\le(m_im_jm_km_\ell)^{1/4}\\ &\le\tfrac14(m_i+m_j+m_k+m_\ell). \end{aligned}

Finally, summing these bounds and using the fixed-position count yields

Var⁡(T)≤14∑(i,j,k,ℓ)∈Q(mi+mj+mk+mℓ)≤4D3∑imi≤13D3∑imi,2πVar⁡(V)≤26π D3/2σ2(∑imi)1/2.\begin{aligned} \operatorname{Var}(T) &\le\frac14\sum_{(i,j,k,\ell)\in\mathcal Q} (m_i+m_j+m_k+m_\ell)\\ &\le4D^3\sum_i m_i \le13D^3\sum_i m_i,\\ \sqrt{\frac2\pi}\sqrt{\operatorname{Var}(V)} &\le\sqrt{\frac{26}{\pi}}\, \frac{D^{3/2}}{\sigma^2}\left(\sum_i m_i\right)^{1/2}. \end{aligned}

Combining the two error bounds proves Theorem 1. The direct covariance count under our full graph assumption actually permits 8/π\sqrt{8/\pi} in place of 26/π\sqrt{26/\pi}; we retained the latter to display the source theorem's constant. Neither constant is asserted optimal.

4. When does the bound imply a CLT?

For a triangular array {Xni:1≤i≤n}\{X_{ni}:1\le i\le n\}, let DnD_n, σn\sigma_n, and WnW_n denote the corresponding quantities. Theorem 1 gives

Dn2σn3∑iE∣Xni∣3⟶0,Dn3/2σn2(∑iEXni4)1/2⟶0⟹Wn⇒N(0,1).\begin{gathered} \frac{D_n^2}{\sigma_n^3}\sum_i\mathbb E|X_{ni}|^3\longrightarrow0,\\ \frac{D_n^{3/2}}{\sigma_n^2} \left(\sum_i\mathbb E X_{ni}^4\right)^{1/2}\longrightarrow0 \end{gathered} \quad\Longrightarrow\quad W_n\Rightarrow N(0,1).

For a simple sufficient condition, assume uniformly bounded third and fourth moments and σn2≥cn\sigma_n^2\ge cn for some fixed c>0c>0. Then

dW(Wn,Z)≤C(Dn2n+Dn3/2n),d_{\mathrm W}(W_n,Z) \le C\left(\frac{D_n^2}{\sqrt n} +\frac{D_n^{3/2}}{\sqrt n}\right),

where CC depends only on the moment bounds and cc. In particular,

Dn=o(n1/4)⟹Wn⇒N(0,1).D_n=o(n^{1/4})\quad\Longrightarrow\quad W_n\Rightarrow N(0,1).

This is a sufficient condition from this bound, not a necessary degree threshold. The variance assumption must be verified separately; local dependence allows negative as well as positive covariances.

Example: Overlapping observations on a path

Let ε1,…,εn+1\varepsilon_1,\ldots,\varepsilon_{n+1} be independent random signs, each taking −1-1 and 11 with equal probability, and set

Xi=εi+εi+1,1≤i≤n.X_i=\varepsilon_i+\varepsilon_{i+1},\qquad 1\le i\le n.

Only adjacent observations share an input. The path graph is a dependency graph, with D≤3D\le3. Direct calculation gives

Var⁡(Xi)=2,Cov⁡(Xi,Xi+1)=1,Cov⁡(Xi,Xj)=0(∣i−j∣>1),σ2=2n+2(n−1)=4n−2.\begin{aligned} \operatorname{Var}(X_i)&=2,\\ \operatorname{Cov}(X_i,X_{i+1})&=1,\\ \operatorname{Cov}(X_i,X_j)&=0\quad(|i-j|>1),\\ \sigma^2&=2n+2(n-1)=4n-2. \end{aligned}

Because XiX_i equals −2,0,2-2,0,2 with probabilities 1/4,1/2,1/41/4,1/2,1/4,

E∣Xi∣3=4,EXi4=8.\mathbb E|X_i|^3=4,\qquad \mathbb E X_i^4=8.

Thus Theorem 1 implies the explicit bound

dW(S4n−2,Z)≤36n(4n−2)3/2+26π 33/28n4n−2=O(n−1/2).d_{\mathrm W}\left(\frac{S}{\sqrt{4n-2}},Z\right) \le\frac{36n}{(4n-2)^{3/2}} +\sqrt{\frac{26}{\pi}}\, \frac{3^{3/2}\sqrt{8n}}{4n-2} =O(n^{-1/2}).

The normalization can also be checked by collecting the independent inputs:

S=ε1+2∑i=2nεi+εn+1.S=\varepsilon_1+2\sum_{i=2}^n\varepsilon_i+\varepsilon_{n+1}.

Ignoring the adjacent covariances would give variance 2n2n. By Slutsky's theorem, the resulting incorrectly normalized sum instead satisfies

S2n=4n−22n Wn⇒N(0,2),\frac{S}{\sqrt{2n}} =\sqrt{\frac{4n-2}{2n}}\,W_n \Rightarrow N(0,2),

not a standard normal limit.

The sign of the shared input also matters. Replacing XiX_i by εi−εi+1\varepsilon_i-\varepsilon_{i+1} preserves the path dependency graph, but telescoping gives

S=ε1−εn+1,σ2=2,P(S/σ=0)=12.S=\varepsilon_1-\varepsilon_{n+1},\qquad \sigma^2=2,\qquad \mathbb P(S/\sigma=0)=\tfrac12.

This standardized distribution is the same for every nn and is not normal. Bounded neighborhoods and bounded observations alone therefore do not suffice.

Example: Growing neighborhoods can still permit a CLT

Partition n=mnbnn=m_nb_n observations into mnm_n independent blocks, each of size bnb_n. Within block rr, set every observation equal to a single random sign εr\varepsilon_r. A union of cliques is a dependency graph, and

Dn=bn,Sn=bn∑r=1mnεr,σn2=mnbn2=nbn.D_n=b_n,\qquad S_n=b_n\sum_{r=1}^{m_n}\varepsilon_r,\qquad \sigma_n^2=m_nb_n^2=nb_n.

All third absolute and fourth moments equal one. Substitution, without imposing σn2≍n\sigma_n^2\asymp n, gives

Dn2nσn3=1mn,Dn3/2nσn2=1mn.\begin{aligned} \frac{D_n^2n}{\sigma_n^3} &=\frac1{\sqrt{m_n}},\\ \frac{D_n^{3/2}\sqrt n}{\sigma_n^2} &=\frac1{\sqrt{m_n}}. \end{aligned}

Therefore mn→∞m_n\to\infty suffices, even when bnb_n grows faster than n1/4n^{1/4}. Indeed,

Wn=1mn∑r=1mnεr.W_n=\frac1{\sqrt{m_n}}\sum_{r=1}^{m_n}\varepsilon_r.

At the opposite extreme, one block gives Wn=ε1W_n=\varepsilon_1 for every nn. Increasing the number of copies of the same observation does not create a normal limit.

5. From smooth test functions to distribution functions

For distribution-function error, define the Kolmogorov distance

dK(W,Z)=sup⁡z∈R∣P(W≤z)−Φ(z)∣.d_{\mathrm K}(W,Z) =\sup_{z\in\mathbb R}|\mathbb P(W\le z)-\Phi(z)|.

The Wasserstein bound already implies convergence in this metric. To see this directly, let hz,δh_{z,\delta} equal one on (−∞,z](-\infty,z], decline linearly to zero on [z,z+δ][z,z+\delta], and equal zero afterward. Its Lipschitz constant is 1/δ1/\delta. Since φ≤M=(2π)−1/2\varphi\le M=(2\pi)^{-1/2},

P(W≤z)−Φ(z)≤Ehz,δ(W)−Ehz,δ(Z)+Ehz,δ(Z)−Φ(z)≤dW(W,Z)δ+∫zz+δz+δ−tδφ(t) dt≤dW(W,Z)δ+Mδ2.\begin{aligned} \mathbb P(W\le z)-\Phi(z) &\le\mathbb E h_{z,\delta}(W)-\mathbb E h_{z,\delta}(Z)\\ &\quad+\mathbb E h_{z,\delta}(Z)-\Phi(z)\\ &\le\frac{d_{\mathrm W}(W,Z)}{\delta} +\int_z^{z+\delta}\frac{z+\delta-t}{\delta}\varphi(t)\,dt\\ &\le\frac{d_{\mathrm W}(W,Z)}{\delta}+\frac{M\delta}{2}. \end{aligned}

A ramp on [z−δ,z][z-\delta,z] gives the same bound for the opposite difference. Minimizing over δ>0\delta>0 yields

dK(W,Z)≤2M dW(W,Z)=(2π)1/4dW(W,Z).\boxed{ d_{\mathrm K}(W,Z)\le\sqrt{2M\,d_{\mathrm W}(W,Z)} =\left(\frac2\pi\right)^{1/4}\sqrt{d_{\mathrm W}(W,Z)}. }

If dW=0d_{\mathrm W}=0, take δ↓0\delta\downarrow0. Otherwise the minimizing value is δ=2dW/M\delta=\sqrt{2d_{\mathrm W}/M}. For the path example this gives a Kolmogorov bound of order n−1/4n^{-1/4}, losing a square root relative to the Wasserstein rate.

6. Chen–Shao: A direct Berry–Esseen bound

The preceding smoothing argument turns a Wasserstein rate into a weaker distribution-function rate. To avoid that loss, we can work directly with hz(w)=1{w≤z}h_z(w)=\mathbf1\{w\le z\}. Its Stein solution satisfies

fz′(w)−wfz(w)=1{w≤z}−Φ(z),f_z'(w)-wf_z(w)=\mathbf1\{w\le z\}-\Phi(z),

with the derivative at zz defined by this equation. The derivative now has an indicator discontinuity, so the earlier global Lipschitz bound on fz′f_z' cannot be used. In the integral remainder, the indicator part satisfies

∣∫0η{1{W−t≤z}−1{W≤z}} dt∣≤∣η∣ 1{∣W−z∣≤∣η∣}.\left|\int_0^\eta \{\mathbf1\{W-t\le z\}-\mathbf1\{W\le z\}\}\,dt\right| \le|\eta|\,\mathbf1\{|W-z|\le|\eta|\}.

The new task is to control probability in a short interval around zz, including intervals whose widths depend on the local variables. This is the role of the concentration inequalities in Chen and Shao (2004), Sections 3–4.

Nested local-dependence conditions

Continue to use normalized summands ξi=Xi/σ\xi_i=X_i/\sigma, so that Var⁡(∑iξi)=1\operatorname{Var}(\sum_i\xi_i)=1. For a set AA, write ξA=(ξj)j∈A\xi_A=(\xi_j)_{j\in A} and let AcA^c denote its complement in {1,…,n}\{1,\ldots,n\}. The local-dependence conditions are

LD1:ξi⊥ ⁣ ⁣ ⁣⊥ξAic,LD2:LD1 andξAi⊥ ⁣ ⁣ ⁣⊥ξBic,LD3:LD2 andξBi⊥ ⁣ ⁣ ⁣⊥ξCic,i∈Ai⊆Bi⊆Ci.\begin{array}{ll} \mathrm{LD1}:&\xi_i\perp\!\!\!\perp\xi_{A_i^c},\\[3pt] \mathrm{LD2}:&\mathrm{LD1}\ \text{and}\quad \xi_{A_i}\perp\!\!\!\perp\xi_{B_i^c},\\[3pt] \mathrm{LD3}:&\mathrm{LD2}\ \text{and}\quad \xi_{B_i}\perp\!\!\!\perp\xi_{C_i^c}, \end{array} \qquad i\in A_i\subseteq B_i\subseteq C_i.

The conditions successively require independence for an observation, its first neighborhood, and a larger block. A dependency graph supplies these separations by taking

Ai=Ni,Bi=⋃j∈AiNj,Ci=⋃j∈BiNj.A_i=N_i,\qquad B_i=\bigcup_{j\in A_i}N_j,\qquad C_i=\bigcup_{j\in B_i}N_j.

For general local-dependence models, LD1 alone does not justify these larger-block independence statements.

Theorem 2: Chen–Shao’s uniform bound

Assume LD3 and finite ppth absolute moments for some 2<p≤32<p\le3. Define

N(Ci)={j:Bj∩Ci≠∅},κ=max⁡imax⁡{∣N(Ci)∣, ∣{j:i∈Cj}∣}.\mathcal N(C_i)=\{j:B_j\cap C_i\ne\varnothing\},\qquad \kappa=\max_i\max\left\{ |\mathcal N(C_i)|,\ |\{j:i\in C_j\}| \right\}.

Then Chen–Shao, Theorem 2.4 gives

dK(W,Z)≤75κp−1∑iE∣ξi∣p=75κp−1σp∑iE∣Xi∣p.\boxed{ d_{\mathrm K}(W,Z) \le75\kappa^{p-1}\sum_i\mathbb E|\xi_i|^p =\frac{75\kappa^{p-1}}{\sigma^p} \sum_i\mathbb E|X_i|^p. }

Unlike the fourth-moment graph bound, this result controls distribution functions directly and permits p≤3p\le3. Its neighborhood parameter κ\kappa differs from DD. It also counts how enlarged neighborhoods intersect and how many of them contain a given index, as specified above. We quote the theorem rather than reproduce its proof. The indicator-remainder calculation explains why the earlier Taylor argument alone is insufficient.

For the path example, choose AiA_i, BiB_i, and CiC_i to have radii one, two, and three. Then

N(Ci)⊆{j:∣j−i∣≤5},∣N(Ci)∣≤11,{j:i∈Cj}⊆{j:∣j−i∣≤3},∣{j:i∈Cj}∣≤7.\begin{aligned} \mathcal N(C_i)&\subseteq\{j:|j-i|\le5\}, &|\mathcal N(C_i)|&\le11,\\ \{j:i\in C_j\}&\subseteq\{j:|j-i|\le3\}, &|\{j:i\in C_j\}|&\le7. \end{aligned}

Taking p=3p=3, κ≤11\kappa\le11, σ2=4n−2\sigma^2=4n-2, and E∣Xi∣3=4\mathbb E|X_i|^3=4 therefore gives

dK(S4n−2,Z)≤75⋅112⋅4n(4n−2)3/2=O(n−1/2).d_{\mathrm K}\left(\frac{S}{\sqrt{4n-2}},Z\right) \le\frac{75\cdot11^2\cdot4n}{(4n-2)^{3/2}} =O(n^{-1/2}).

The rate improves on the n−1/4n^{-1/4} obtained by smoothing our Wasserstein bound, although the displayed numerical constant is large. This is a comparison for this example, not a claim that every Chen–Shao bound is numerically sharper than every graph bound.

More generally, if κn\kappa_n is bounded, sup⁡n,iE∣Xni∣p<∞\sup_{n,i}\mathbb E|X_{ni}|^p<\infty, and σn2≥cn\sigma_n^2\ge cn, then

dK(Wn,Z)≤Cn1−p/2⟶0.d_{\mathrm K}(W_n,Z) \le C n^{1-p/2}\longrightarrow0.

7. What is still needed for statistical inference?

The earlier doubly robust note separates a population identity from estimation error. A dependence CLT requires the same separation. For a target θn\theta_n, suppose an estimator θ^n\widehat\theta_n admits

n(θ^n−θn)=∑i=1nXni+rn,rn=oP(σn),n(\widehat\theta_n-\theta_n) =\sum_{i=1}^nX_{ni}+r_n, \qquad r_n=o_{\mathbb P}(\sigma_n),

with centered leading summands satisfying one of the preceding CLTs. Then

n(θ^n−θn)σn=Wn+oP(1)⇒N(0,1).\frac{n(\widehat\theta_n-\theta_n)}{\sigma_n} =W_n+o_{\mathbb P}(1) \Rightarrow N(0,1).

If, in addition, a positive estimator σ^n\widehat\sigma_n of σn\sigma_n satisfies σ^n/σn→P1\widehat\sigma_n/\sigma_n\to_{\mathbb P}1, Slutsky's theorem gives

n(θ^n−θn)σ^n⇒N(0,1).\frac{n(\widehat\theta_n-\theta_n)}{\widehat\sigma_n} \Rightarrow N(0,1).

The dependency graph must describe these leading summands, not just the observed network. Shared fitted nuisance functions can introduce dependence beyond graph neighbors; a separate expansion and remainder argument is then required. Likewise, the CLT does not itself prove consistency of σ^n\widehat\sigma_n.

For a random graph, the deterministic-graph theorem can instead be applied conditionally when its centering, independence, moment, and positive-variance conditions hold given that graph. The conditional approximation error must tend to zero, and the normalization must use the corresponding conditional variance. Randomness of the graph alone does not verify those conditions.

References

  • Nathan Ross (2011). Fundamentals of Stein’s method. Lemma 2.5 supplies the Stein factors; Section 3.2, Theorem 3.5, supplies the dependency-neighborhood bound.
  • Louis H. Y. Chen and Qi-Man Shao (2004). Normal approximation under local dependence. The Annals of Probability, 32(3A), 1985–2028. Section 2 defines LD1–LD3; Theorem 2.4 gives the uniform Berry–Esseen bound used here, with concentration inequalities and proofs in Sections 3–4.