Law of Large Numbers

Law of Large Numbers • Toss a coin n times. • Suppose • Xi’s are Bernoulli random variables with p = ½ and E(Xi) = ½. • The proportion of heads is . • Intuitively approaches ½ as n  ∞. week 12

Markov’s Inequality • If X is a non-negative random variable with E(X) < ∞ and a >0 then, week 12

Chebyshev’s Inequality • For a random variable X with E(X) < ∞ and V(X) < ∞, for any a >0 • Proof: week 12

Back to the Law of Large Numbers • Interested in sequence of random variables X1, X2, X3,… such that the random variables are independent and identically distributed (i.i.d). Let Suppose E(Xi) = μ , V(Xi) = σ2, then and • Intuitively, as n  ∞, so week 12

Formally, the Weak Law of Large Numbers (WLLN) states the following: • Suppose X1, X2, X3,…are i.i.d with E(Xi) = μ < ∞ , V(Xi) = σ2 < ∞, then for any positive number a as n ∞ . This is called Convergence in Probability. Proof: week 12

Example • Flip a coin 10,000 times. Let • E(Xi) = ½ and V(Xi) = ¼ . • Take a = 0.01, then by Chebyshev’s Inequality • Chebyshev Inequality gives a very weak upper bound. • Chebyshev Inequality works regardless of the distribution of the Xi’s. week 12

Strong Law of Large Number • Suppose X1, X2, X3,…are i.i.d with E(Xi) = μ < ∞ , then converges to μ as n  ∞ with probability 1. That is • This is called convergence almost surely. week 12

Continuity Theorem for MGFs • Let X be a random variable such that for some t0 > 0 we have mX(t) < ∞ for . Further, if X1, X2,…is a sequence of random variables with and for all then {Xn} converges in distribution to X. • This theorem can also be stated as follows: Let Fn be a sequence of cdfs with corresponding mgf mn. Let F be a cdf with mgf m. If mn(t) m(t) for all t in an open interval containing zero, then Fn(x) F(x) at all continuity points of F. • Example: Poisson distribution can be approximated by a Normal distribution for large λ. week 12

Example to illustrate the Continuity Theorem • Let λ1, λ2,…be an increasing sequence with λn∞ as n ∞ and let {Xi} be a sequence of Poisson random variables with the corresponding parameters. We know that E(Xn) = λn = V(Xn). • Let then we have that E(Zn) = 0, V(Zn) = 1. • We can show that the mgf of Zn is the mgf of a Standard Normal random variable. • We say that Zn convergence in distribution to Z ~ N(0,1). week 12

Example • Suppose X is Poisson(900) random variable. Find P(X > 950). week 12

Central Limit Theorem • The central limit theorem is concerned with the limiting property of sums of random variables. • If X1, X2,…is a sequence of i.i.d random variables with mean μ and variance σ2 and , then by the WLLN we have that in probability. • The CLT concerned not just with the fact of convergence but how Sn/n fluctuates around μ. • Note that E(Sn) = nμ and V(Sn) = nσ2. The standardized version of Sn is and we have that E(Zn) = 0, V(Zn) = 1. week 12

The Central Limit Theorem • Let X1, X2,…be a sequence of i.i.d random variables with E(Xi) = μ < ∞ and Var(Xi) = σ2 < ∞. Suppose the common distribution function FX(x) and the common moment generating function mX(t) are defined in a neighborhood of 0. Let Then, for - ∞ < x < ∞ where Ф(x) is the cdf for the standard normal distribution. • This is equivalent to saying that converges in distribution to Z ~ N(0,1). • Also, i.e. converges in distribution to Z ~ N(0,1). week 12

Example • Suppose X1, X2,…are i.i.d random variables and each has the Poisson(3) distribution. So E(Xi) = V(Xi) = 3. • The CLT says that as n  ∞. week 12

Examples • A very common application of the CLT is the Normal approximation to the Binomial distribution. • Suppose X1, X2,…are i.i.d random variables and each has the Bernoulli(p) distribution. So E(Xi) = p and V(Xi) = p(1- p). • The CLT says that as n  ∞. • Let Yn = X1 + … + Xn then Yn has a Binomial(n, p) distribution. So for large n, • Suppose we flip a biased coin 1000 times and the probability of heads on any one toss is 0.6. Find the probability of getting at least 550 heads. • Suppose we toss a coin 100 times and observed 60 heads. Is the coin fair? week 12

Law of Large Numbers