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## 5.1. Jointly Distributed Random Variables

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**Chapter 5**Joint Probability Distributions and Random Samples 5.1. Jointly Distributed Random Variables**Joint Probability Mass Function**Let X and Y be two discrete rv’s defined on the sample space of an experiment. The joint probability mass functionp(x, y) is defined for each pair of numbers (x, y) by Let A be the set consisting of pairs of (x, y) values, then**Marginal Probability Mass Functions**The marginal probability mass functions of X and Y, denoted pX(x) and pY(y) are given by**Joint Probability Density Function**Let X and Y be continuous rv’s. Then f (x, y) is a joint probability density function for X and Y if for any two-dimensional set A If A is the two-dimensional rectangle**A = shaded rectangle**= Volume under density surface above A**Marginal Probability Density Functions**The marginal probability density functionsof X and Y, denoted fX(x) and fY(y), are given by**Independent Random Variables**Two random variables X and Y are said to be independent if for every pair of x and y values when X and Y are discrete or when X and Y are continuous. If the conditions are not satisfied for all (x, y) then X and Y are dependent.**Multivariate Random Variables**If X1, X2,…, Xn are all discrete random variables, the joint pmf of the variables is the function If the variables are continuous, the joint pdf is the function f such that for any n intervals [a1,b1], …,[an,bn],**Independence – More Than Two Random Variables**The random variables X1, X2,…,Xn are independent if for every subset of the variables, the joint pmf or pdf of the subset is equal to the product of the marginal pmf’s or pdf’s.**Conditional Distributions:**Definition: Let (X, Y) be a discrete r.v. with jpmf, p(xi, yj ).The conditional pmf’s are defined by P(X = xiY=yj)= PX|Y( xiyj )= p(xi,yj) / pY(yj) pY(yj) (0)**Conditional Probability Functions**Let X and Y be two continuous rv’s with joint pdf f (x, y) and marginal X pdf fX(x). Then for any X value x for which fX(x) > 0, the conditional probability density function of Y given that X = x is If X and Y are discrete, replacing pdf’s by pmf’s gives the conditional probability mass function of Y when X = x.**Conditional Expectation and Variance**• The conditional expectation of a r.v. X given that Y=y is defined in the discrete case by • and in the continuous case by**Example**• Let (X, Y) ~ f(x, y)= (1/2) e-x, x 0, |y|<x • Furthermore, the conditional variance of X given Y = y is defined by • V(XY= y) = E(X2 Y = y) - [E(XY = y) ]2.**5.2 Expected Values, Covariance &Correlation**Let X and Y be jointly distributed rv’s with pmf p(x, y) or pdf f (x, y) according to whether the variables are discrete or continuous. Then the expected value of a function h(X, Y), denoted E[h(X, Y)] or is discrete continuous**Covariance**The covariance between two rv’s X and Y is discrete continuous**Correlation**The correlation coefficient of X and Y, denoted by Corr(X, Y), defined by**Correlation Proposition**• If a and c are either both positive or both negative, Corr(aX + b, cY + d) = Corr(X, Y) • For any two rv’s X and Y,**Correlation Proposition**• If X and Y are independent, then but does not imply independence. • for some numbers a and b with**5.3**Statistics and their Distributions**Statistic**A statistic is a function of random variables. e.g Where (X1, X2, …, Xn), are r.v.’s. An observations of this vector of r.v.’s from a specific random sample is denoted by ( x1, x2, …, xn), in this case is a specific value of**Random Samples**• The rv’s X1,…,Xn are said to form a (simple • random sample of size n if • The Xi’s are independent rv’s. • Every Xi has the same probability distribution.**Simulation Experiments**The following characteristics must be specified: • The statistic of interest. • The population distribution. • The sample size n. • The number of replications k.**5.4**The Distribution of the Sample Mean**Using the Sample Mean**Let X1,…, Xn be a random sample from a distribution with mean value and standard deviation Then In addition, with To = X1 +…+ Xn,**Normal Population Distribution**Let X1,…, Xn be a random sample from a normal distribution with mean value and standard deviation Then for any n, is normally distributed, as is To.**The Central Limit Theorem**Let X1,…, Xn be a random sample from a distribution with mean value and variance Then if n sufficiently large, has approximately a normal distribution with and To also has approximately a normal distribution with The larger the value of n, the better the approximation.**The Central Limit Theorem**small to moderate n large n Population distribution**Rule of Thumb**If n > 30, the Central Limit Theorem can be used.**Approximate Lognormal Distribution**Let X1,…, Xn be a random sample from a distribution for which only positive values are possible [P(Xi > 0) = 1]. Then if n is sufficiently large, the product Y = X1X2…Xn has approximately a lognormal distribution.**5.5**The Distribution of a Linear Combination**Linear Combination**Given a collection of n random variables X1,…, Xn and n numerical constants a1,…,an, the rv is called a linear combination of the Xi’s.**Expected Value of a Linear Combination**Let X1,…, Xn have mean values and variances of respectively Whether or not the Xi’s are independent,**Variance of a Linear Combination**If X1,…, Xn are independent, and**Variance of a Linear Combination**For any X1,…, Xn,**Difference Between Two Random Variables**and, if X1 and X2 are independent,**Difference Between Normal Random Variables**If X1, X2,…Xnare independent, normally distributed rv’s, then any linear combination of the Xi’s also has a normal distribution. The difference X1 – X2 between two independent, normally distributed variables is itself normally distributed.