Isotonic regression: Difference between revisions

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In [[probability theory]], the [[central limit theorem]] says that, under certain conditions, the sum of many [[independent identically-distributed random variables]], when scaled appropriately, [[converges in distribution]] to a standard [[normal distribution]]. The '''martingale central limit theorem''' generalizes this result for random variables to [[Martingale (probability theory)|martingale]]s, which are [[stochastic process]]es where the change in the value of the process from time ''t'' to time ''t''&nbsp;+&nbsp;1 has [[expected value|expectation]] zero, even conditioned on previous outcomes.
 
==Statement==
Here is a simple version of the martingale central limit theorem: Let
:<math>X_1, X_2, \dots\,</math> -- be a martingale with bounded increments, i.e., suppose
 
:<math>\operatorname{E}[X_{t+1} -  X_t \vert X_1,\dots, X_t]=0\,,</math>
 
and
 
:<math>|X_{t+1} - X_t| \le k</math>
 
[[almost surely]] for some fixed bound ''k'' and all ''t''. Also assume that <math>|X_1|\le k</math> almost surely.
 
Define
 
:<math>\sigma_t^2 = \operatorname{E}[(X_{t+1}-X_t)^2|X_1, \ldots, X_t],</math>
 
and let
 
:<math>\tau_\nu = \min\left\{t : \sum_{i=1}^{t} \sigma_i^2 \ge \nu\right\}.</math>
 
Then
 
:<math>\frac{X_{\tau_\nu}}{\sqrt{\nu}}</math>
 
converges in distribution to the normal distribution with mean 0 and variance 1 as <math>\nu \to +\infty \!</math>. More explicitly,
 
:<math>\lim_{\nu \to +\infty} \operatorname{P} \left(\frac{X_{\tau_\nu}}{\sqrt{\nu}} < x\right) = \Phi(x)
= \frac{1}{\sqrt{2\pi}}
\int_{-\infty}^x
\exp\left(-\frac{u^2}{2}\right)
\, du, \quad x\in\mathbb{R}.
</math>
 
==References==
Many other variants on the martingale central limit theorem can be found in:
*{{cite book | first = Peter | last = Hall | coauthors = and C. C. Heyde | year = 1980 | title = Martingale Limit Theory and Its Application | publisher = Academic Press | location = New York | isbn = 0-12-319350-8}}
 
*For the discussion of Theorem 5.4 there, and correct form of Corollary 5.3(ii), see {{cite journal| 
last=Bradley| first=Richard|
  journal=Journal of Theoretical Probability|
  volume=1|
  pages=115–119|
  year=1988|
  publisher=Springer | title=On some results of MI Gordin: a clarification of a misunderstanding|
doi=10.1007/BF01046930| 
issue=2}}
 
[[Category:Martingale theory]]
[[Category:Central limit theorem]]

Revision as of 12:07, 24 July 2013

In probability theory, the central limit theorem says that, under certain conditions, the sum of many independent identically-distributed random variables, when scaled appropriately, converges in distribution to a standard normal distribution. The martingale central limit theorem generalizes this result for random variables to martingales, which are stochastic processes where the change in the value of the process from time t to time t + 1 has expectation zero, even conditioned on previous outcomes.

Statement

Here is a simple version of the martingale central limit theorem: Let

X1,X2, -- be a martingale with bounded increments, i.e., suppose
E[Xt+1Xt|X1,,Xt]=0,

and

|Xt+1Xt|k

almost surely for some fixed bound k and all t. Also assume that |X1|k almost surely.

Define

σt2=E[(Xt+1Xt)2|X1,,Xt],

and let

τν=min{t:i=1tσi2ν}.

Then

Xτνν

converges in distribution to the normal distribution with mean 0 and variance 1 as ν+. More explicitly,

limν+P(Xτνν<x)=Φ(x)=12πxexp(u22)du,x.

References

Many other variants on the martingale central limit theorem can be found in:

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