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Suppose random variable $X$ has a continuous probability distribution with an unbounded upper tail; that is, the CDF of $X$ (call it $F$) is absolutely continuous and $F(x)<1$ for all $x\in\mathbb{R}.$

Now consider the conditional CDF of ${X-a\over w}$ given that $a\le X\le a+w$, for $w>0$:$$G(y,a,w):=\mathbb{P}\left({X-a\over w}\le y\ {\LARGE \mid}\ a\le X\le a+w\right) ={F(a+w\,y)-F(a)\over F(a+w)-F(a)}\,1_{0\le y\le 1}+1_{y>1}.$$

WolframCloud computations lead to the following ...

Observation: Apparently, for all well-known parametric families of such distributions (e.g., Normal, Lognormal, Student's t, Cauchy, Maxwell, Gamma, Gumbel, Weibull, etc.), there are only three types of limit of the CDF of ${X-a\over w}\mid a\le X\le a+w$ as $a\to\infty$: $$\lim_{a\to\infty}G(y,a,w)=\lim_{a\to\infty}\mathbb{P}\left({X-a\over w}\le y\ {\LARGE \mid}\ a\le X\le a+w\right) \\[3ex] = \begin{cases} 1_{y>0} & \text{(Type 1: Degenerate at $0$)}\\[2ex] y\,1_{0\le y\le 1}+1_{y>1} & \text{(Type 2: Uniform on $[0,1]$)}\\[2ex] {1- e^{-y\,w/\beta}\over 1-e^{-w/\beta}}1_{0\le y\le 1}+1_{y>1} & \text{(Type 3: Exponential$(\beta/w)$ on $[0,1]$)} \end{cases}$$ where $\beta$ is a parameter depending on the $X$ distribution.

NB: The Type 3 limit depends on both $w$ and the $X$ distribution, but the Type 1 and Type 2 limits are free of w and free of all parameters of the $X$ distribution, which seems quite remarkable.

NB: Letting $Y_a={X-a\over w}$, for all three types (and any other type, if there are any) we have $$\begin{align}y\le 0&\implies \{Y_a\le y\}\cap\{0\le Y_a\le 1\}=\{Y_a=0\}\implies \lim_{a\to\infty}G(y,a,w)=0\\ y\ge 1&\implies \{Y_a\le y\}\cap\{0\le Y_a\le 1\}=\{0\le Y_a\le 1\}\implies \lim_{a\to\infty}G(y,a,w)=1\end{align}$$ since $\mathbb{P}(Y_a=0)=0$ due to the distribution of $X$ (hence $Y_a$) being continuous; thus, the types of limit differ only for $y$ in the open interval $(0,1)$.

Here are pictures of these three types of limit: ![enter image description here

Questions:

  1. How generally does this observation hold, and how can it be proved analytically? (Is this something well-known?)

  2. What $X$ distributions (if any) do in fact yield a limit not among these three types?

  3. In the Degenerate case, the limit is not right-continuous at $y=0$, so it fails to be a CDF. Nevertheless, as $a\to\infty$ the probability mass clearly concentrates in an arbitrarily small neighborhood of $y=0$ (as indicated by the plots below); so, is there a valid interpretation of this as a limiting distribution?

(My original motivating question concerned only the asymptotic variance for just Normal vs. Lognormal. These variances of course follow from the above limiting distributions.)


Examples

Degenerate Limit

Here are the CDFs (left) and corresponding PDFs (right) for Normal($0,1$), showing the CDFs, PDFs labeled as $G(y,a,w),\ g(y,a,w)$ for various $a$ with $w=1$: enter image description here Computations show that the limit of the distribution is degenerate at $0$ for apparently any Normal, Gumbel, Rayleigh or Maxwell distribution, or any Weibull distribution with shape parameter $> 1.$

Uniform Limit

Here are the CDFs (left) and corresponding PDFs (right) for LogNormal(0,1), showing the CDFs, PDFs labeled as $G(y,a,w),\ g(y,a,w)$ for various $a$ with $w=1$: enter image description here Computations show that the limit distribution is Uniform on $[0,1]$ for apparently any LogNormal, Student-t, Cauchy, Levi, Pareto, or Inverse Gamma distribution, or any Weibull distribution with shape parameter $< 1.$

Exponential Limit

Here are the CDFs (left) and corresponding PDFs (right) for Gamma($\alpha=2,\beta=3$), showing the CDFs, PDFs labeled as $G(y,a,w),\ g(y,a,w)$ for various $a$ with $w=1$: ![enter image description here

Computations show that the limit distribution is Exponential on $[0,1]$ for apparently any Gamma distribution (which includes the Weibull distribution with shape parameter $= 1$), or any Logistic, Laplace, or Extreme Value distribution.


For reference, here's the Wolfram code I used:

(* Find limit(a->infinity) Pr[(X-a)/w <= y | a<=X<=a+w] for 0<y<1 *)
FindLimit[distr_,asms_]:=Module[{assms=asms~Join~{0<y<1,a>0,w>0}}, 
G[y_,a_,w_]:=Probability[(X-a)/w<=y\[Conditioned]a<=X<=a+w,X\[Distributed]distr];
Glim[y_,w_]:=Simplify[Cancel[Limit[G[y,a,w],a->Infinity]]];
Print["0<y<1: ", Assuming[assms,Glim[y,w]]];] 

(* Examples: *)

(* In[]= *)
FindLimit[ NormalDistribution[\[Mu],\[Sigma]],{\[Mu]\[Element]Reals ,\[Sigma]>0} ] 
(* Out[]= *)
0<y<1: 1

(* In[]= *)
FindLimit[ LogNormalDistribution[\[Mu],\[Sigma]],{\[Mu]\[Element]Reals ,\[Sigma]>0} ] 
(* Out[]= *)
0<y<1: y

(* In[]= *)
FindLimit[ GammaDistribution[\[Alpha],\[Beta]],{\[Alpha]>0,\[Beta]>0} ]
(* Out[]= *)
0<y<1: (E^(w/\[Beta]) (1-E^(-((w y)/\[Beta]))))/(-1+E^(w/\[Beta]))
$\endgroup$
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  • $\begingroup$ As you say, the CDF of your conditional distribution is given by $G(x)=\frac{F(a+x)-F(a)}{F(a+1)-F(a)}$, where $F$ is the CDF of $X$. Since $F$ could be any right-continuous non-decreasing function going from $0$ to $1$ on $\mathbb R$, it would seem like you could get a very large class of possible functions $G$ if you allow any CDF $F$. $\endgroup$
    – Jack M
    Commented Sep 8, 2018 at 18:47
  • $\begingroup$ @JackM It is a large class, even though $F$ must be absolutely continuous with $F(x)<1$ for all $x$. But in this class, I haven't been able to find any examples that do not have one of the three stated types of limit. $\endgroup$
    – r.e.s.
    Commented Sep 9, 2018 at 17:23
  • $\begingroup$ Have you proven that these distributions yield only the three types of limits you mention, or have you only done numerical tests? $\endgroup$
    – Jack M
    Commented Sep 9, 2018 at 17:47
  • $\begingroup$ @JackM I do not have proof, just the Wolfram results for numerous specific distributions -- which is why I ask for "analytical proof" in my first question. (But I suppose the limit computations done by the computer algebra system are not mere "numerical tests".) $\endgroup$
    – r.e.s.
    Commented Sep 9, 2018 at 18:18
  • $\begingroup$ I know Gnedenko had done some work in this area on distribution tails. Have you looked at his papers. $\endgroup$ Commented Sep 13, 2018 at 14:22

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