# Convergence and Asymptotic Results

September 24, 2015
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Last week, in our mathematical statistics course, we’ve seen the law of large numbers (that was proven in the probability course), claiming that

$\overline{X}_n\ \xrightarrow{\text{a.s.}}\ \mathbb{E}(X)$

given a collection $\{X_1,\cdots,X_n\}$ of i.i.d. random variables, with

$\overline{X}_n=\frac1n(X_1+\cdots+X_n)$

To visualize that convergence, we can use

```> m=100
> mean_samples=function(n=10){
+   X=matrix(rnorm(n*m),nrow=m,ncol=n)
+   return(apply(X,1,mean))
+ }
> B=matrix(NA,100,20)
> for(i in 1:20){
+   B[,i]=mean_samples(i*10)
+ }
> colnames(B)=as.character(seq(10,200,by=10))
> boxplot(B)```

It is possible to visualize also the $\sqrt{n}$ bounds (used in the central limit theorem to get a limiting non degenerated distribution)

```> u=seq(0,21,by=.2)
> v=sqrt(u*10)
> lines(u,1.96/v,col="red")
> lines(u,-1.96/v,col="red")```

Yesterday, we’ve been discussing properties of the empirical cumulative distribution function,

$\hat F_n(x)=\frac{1}{n}\sum_{i=1}^n \boldsymbol{1}(X_i\in(-\infty,x])$

We’ve seen Glivenko-Cantelli theorem, which states that (under mild assumptions)

$\|\hat F_n-F\|_\infty \equiv \sup_{t\in\mathbb{R}} \big|\hat F_n(t)-F(t)\big|\ \xrightarrow{a.s.}\ 0.$

To visualize that convergence use the following code. Here I use the trick

$\max\{a,b\}=\frac{a+b}{2}+\frac{\vert b-a\vert}{2}$

to get the maximum (componentwise) between two matrices

```> m=100
> inf_sample=function(n=10){
+ X=matrix(rnorm(n*m),nrow=m,ncol=n)
+ Xs=t(apply(X,1,sort))
+ Pe_inf=matrix(rep((0:(n-1))/n,
+ each=m),nrow=m,ncol=n)
+ Pe_sup=matrix(rep((0:n)/n,each=m),
+ nrow=m,ncol=n)
+ Pt=pnorm(Xs)
+ D1=abs(Pe_inf-Pt)
+ D2=abs(Pe_sup-Pt)
+ Df=(D1+D2)/2+abs(D2-D1)/2
+ return(apply(Df,1,max))
+ }
> B=matrix(NA,100,20)
> for(i in 1:20){
+   B[,i]=inf_sample(i*10)
+ }
> colnames(B)=as.character(seq(10,200,by=10))
> boxplot(B)```

We have also discussed the pointwise asymptotic normality of the empirical cumulative distribution function

$\sqrt{n}\big(\hat F_n(t) - F(t)\big)\ \ \xrightarrow{\mathcal{L}}\ \ \mathcal{N}\Big( 0, F(t)\big(1-F(t)\big) \Big).$

Here again, it is possible to visualize it. The first step is to compute several trajectories for empirical cumulative distribution function

```> u=seq(-3,3,by=.1)
> plot(u,u,ylim=c(0,1),col="white")
> M=matrix(NA,length(u),1000)
> for(m in 1:1000){
+ n=100
+ x=rnorm(n)
+ Femp=Vectorize(function(t) mean(x<=t))
+ v=Femp(u)
+ M[,m]=v
+ lines(u,v,col='light blue',type="s")
+ }```

Note that we can compute (pointwise) confidence bands

```> lines(u,apply(M,1,mean),col="red",type="l")
> lines(u,apply(M,1,function(x) quantile(x,.05)),
+ col="red",type="s")
> lines(u,apply(M,1,function(x) quantile(x,.95)),
+ col="red",type="s")```

Now, if we focus on one specific point, we can visualize the asmptotic normality (i.e. the almost normality when we have a sample of size 100)

```> x0=-1
> y=M[which(u==x0),]
> hist(y,probability=TRUE,
+ breaks=seq(.015,0.55,by=.01))
> vu=seq(0,1,by=.001)
> lines(vu,dnorm(vu,pnorm(x0),
+ sqrt((pnorm(x0)*(1-pnorm(x0)))/100)),
+ col="red")```

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