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Theory of Statistics - George Mason University

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0.0 Some Basic Mathematical Concepts 667<br />

simulation, the method relies on the fact that for independent, identically<br />

distributed realizations X1, X2, . . . from the distribution P <strong>of</strong> X,<br />

1<br />

n<br />

n<br />

g(Xi) → Eg(X)<br />

i=1<br />

almost surely as n goes to infinity. This convergence is a simple consequence<br />

<strong>of</strong> the law <strong>of</strong> large numbers.<br />

In Monte Carlo simulation, the sample is simulated with a random number<br />

generator. When X is multivariate or a complicated stochastic process,<br />

however, it may be difficult or impossible to simulate independent realizations.<br />

Monte Carlo Applications<br />

Whether the random number generation is direct or iterative, there are generally<br />

two kinds <strong>of</strong> objectives in Monte Carlo applications. One is just to understand<br />

a probability distribution better. This may involve merely simulating<br />

random observations from the distribution and examining the distribution <strong>of</strong><br />

the simulated sample.<br />

The other main application <strong>of</strong> Monte Carlo methods is to evaluate some<br />

constant. No matter how complicated the problem is, it can always be formulated<br />

as the problem <strong>of</strong> evaluating a definite integral<br />

<br />

f(x)dx.<br />

D<br />

Using a PDF decomposition (0.0.95) f(x) = g(x)p(x), by equation (0.0.96),<br />

we see that the evaluation <strong>of</strong> the integral is the same as the evaluation <strong>of</strong> the<br />

expected value <strong>of</strong> g(X) where X is a random variable whose distribution has<br />

PDF p with support D.<br />

The problem now is to estimate E(g(X)). If we have a sample x1, . . ., xn,<br />

the standard way <strong>of</strong> estimating E(g(X)) is to use<br />

0.0.8 Mathematical Pro<strong>of</strong>s<br />

E(g(X)) = 1<br />

n<br />

n<br />

g(xi). (0.0.72)<br />

A mathematical system consists <strong>of</strong> a body <strong>of</strong> statements, which may be definitions,<br />

axioms, or propositions. A proposition is a conditional statement, which<br />

has the form “if A then B”, or “A ⇒ B”, where A and B are simple declarative<br />

statements or conditional statements. A conditional statement may be true<br />

or false, or neither. Our interest in mathematics is to establish the truth or<br />

falsity <strong>of</strong> a conditional statement; that is, to prove or disprove the statement.<br />

A proposition that has a pro<strong>of</strong> is sometimes called a “lemma”, a “theorem”,<br />

i=1<br />

<strong>Theory</strong> <strong>of</strong> <strong>Statistics</strong> c○2000–2013 James E. Gentle

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