# Topic 4: Multivariate random variables. Multiple random variables

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1 Topic 4: Multivariate random variables Joint, marginal, and conditional pmf Joint, marginal, and conditional pdf and cdf Independence Expectation, covariance, correlation Conditional expectation Two jointly Gaussian random variables ES150 Harvard SEAS 1 Multiple random variables In many problems, we are interested in more than one random variables representing different quantities of interest from the same experiment and the same sample space. Examples: The traffic loads at different routers in a network, the received quality at different HDTVs, the shuttle arrival time at different stations. These random variables can be represented by a random vector X that assign a vector of real number to each outcome s in the sample space Ω. X = (X 1, X 2,..., X n ) It prompts us to investigate the mutual coupling among these random variables. We will study 2 random variables first, before generalizing to a vector of n elements. ES150 Harvard SEAS 2

2 Joint cdf The joint cdf of two random variables X and Y specifies the probability of the event {X x} {Y y} F X,Y (x, y) = P [X x, Y y] The joint cdf is defined for all pairs of random variables. Properties of the joint cdf: Non-negativity: F X,Y (x, y) 0 Non-decreasing: Boundedness: If x 1 x 2 and y 1 y 2, then F X,Y (x 1, y 1 ) F X,Y (x 2, y 2 ) lim F X,Y (x, y) = 1 x,y lim F X,Y (x, y) = lim F X,Y (x, y) = 0 x y ES150 Harvard SEAS 3 Marginal cdf s: which are the individual cdf s F X (x) = lim y F X,Y (x, y) The marginal cdf s can be obtained from the joint cdf, but usually not the reverse. In general, knowledge of all marginal cdf s is insufficient to specify the joint cdf. Rectangle formula: P [a < X b, c < Y d] = F X,Y (b, d) F X,Y (b, c) F X,Y (a, d) + F X,Y (a, c) ES150 Harvard SEAS 4

3 Discrete random variables Joint pmf Consider two discrete random variables X and Y. They are completely specified by their joint pmf, which specifies the probability of the event {X = x, Y = y} p X,Y (x k, y j ) = P (X = x k, Y = y j ) The probability of any event A is given as P ((X, Y ) A) = p X,Y (x k, y j ) By the axioms of probability k=1 j=1 (x k,y j ) A p X,Y (x k, y j ) 0 p X,Y (x k, y j ) = 1 ES150 Harvard SEAS 5 Marginal and conditional pmf s From the joint pmf P X,Y (x, y), we can calculate the individual pmf p X (x) and p Y (y), which are now referred to as the marginal pmf p X (x i ) = p X,Y (x i, y j ) j=1 Similarly p Y (y k ) = i=1 p X,Y (x i, y k ). The marginal pmf is an one-dimensional pmf. In general, knowledge of all marginal pmf s is insufficient to specify the joint pmf. Sometimes we know one of the two random variables, and we are interested in the probability of the other one. This is captured in the conditional pmf p X Y (X = x i Y = y k ) = p X,Y (x i, y k ) p Y (y k ) provided p Y (y k ) 0. Otherwise define p X Y (x y k ) = 0. ES150 Harvard SEAS 6

4 Example: The binary symmetric channel Consider the following binary communication channel Z {0, 1} X {0, 1} Y {0, 1} The bit sent is X Bern(p), 0 p 1 The noise is Z Bern(ɛ), 0 ɛ 0.5 The bit received is Y = (X + Z) mod 2 = X Z where X and Z are independent. Find the following probabilities: 1. p Y (y); 2. p X Y (x y); 3. P [X Y ], the probability of error. ES150 Harvard SEAS 7 Jointly continuous random variables Joint, marginal, and condition pdf Two random variables X and Y are jointly continuous if the probability of any event involving (X, Y ) can be expressed as an integral of a probability density function P [(X, Y ) A] = f X,Y (x, y) dx dy f X,Y (x, y) is called the joint probability density function It is possible to have two continuous random variables that are not jointly continuous. For jointly continuous random variables F X,Y (x, y) = x A y f X,Y (u, v) = 2 F X,Y (x, y) x y f X,Y (u, v) dv du Since F X,Y (x, y) is non-decreasing, the joint density is always ES150 Harvard SEAS 8

5 non-negative f X,Y (x, y) 0 But f X,Y (x, y) is NOT a probability measure (it can be > 1). By the axioms of probability, f X,Y (x, y) dx dy = 1 The marginal pdf can be obtained from the joint pdf as f X (x) = The conditional pdf is given as f X,Y (x, y)dy f X Y (x y) = f X,Y (x, y) f Y (y) Example: Consider two jointly Gaussian random variables with the joint pdf 1 f X,Y (x, y) = { 2π 1 ρ exp x2 2ρxy + y 2 } 2 2(1 ρ 2 ) ES150 Harvard SEAS 9 The marginal pdf of X is Gaussian f X (x) = 1 2π e x2 /2 The conditional pdf of X given Y is also Gaussian { } 1 f X (x y) = 2π(1 ρ2 ) exp (x ρy)2 2(1 ρ 2 ) ES150 Harvard SEAS 10

6 Independence Independence between 2 random variables X and Y means P (X B, Y C) = P (X B) P (Y C) Two random variables X and Y are independent if and only if F X,Y (x, y) = F X (x)f Y (y) Equivalently, in terms of the joint and conditional densities For discrete r.v. s: p X,Y (x i, y k ) = p X (x i )p Y (y k ) p X Y (x i y k ) = p X (x i ) For jointly continuous r.v. s: f X,Y (x, y) = f X (x)f Y (y) f X Y (x y) = f X (x) ES150 Harvard SEAS 11 X and Y independent implies E[XY ] = E[X]E[Y ] The reverse is not always true. That is, E[XY ] = E[X]E[Y ] does not necessarily imply X and Y are independent. Example: Consider discrete RVs X and Y such that P [X = ±1] = P [X = ±2] = 1 4 and Y = X 2 Then E[X] = 0 and E[XY ] = 0, but X and Y are not independent. Find p Y (1), p Y (4), p XY (1, 4). ES150 Harvard SEAS 12

7 Expectation, covariance, correlation Let g(x, y) be a function of two random variables X and Y. The expectation of g(x, Y ) is given by g(x i, y k )p X,Y (x i, y k ) discrete X, Y E[g(X, Y )] = k i g(x, y)f X,Y (x, y) dx dy jointly continuous X, Y The covariance between 2 random variables is defined as cov(x, Y ) = E [ X E[X] ][ Y E[Y ] ] = E[XY ] E[X]E[Y ] If E[X] = 0 or E[Y ] = 0 then cov(x, Y ) = E[XY ]. Covariance is analogous to the variance of a single random variable: cov(x, X) = var(x). E[XY ] is the correlation between the two random variables. By the Cauchy-Schwatz inequality E[XY ] E[X 2 ]E[Y 2 ] ES150 Harvard SEAS 13 X and Y are said to be (note the difference in terminology here) orthogonal if E[XY ] = 0 uncorrelated if cov(x, Y ) = 0 The correlation coefficient is defined as ρ X,Y = cov(x, Y ) σ X σ Y where σ X = var(x) and σ Y = var(y ) are the standard deviations. Properties: Bounded: 1 ρ X,Y 1 If ρ X,Y = 0, X and Y are uncorrelated If X and Y are independent, then ρ X,Y = 0. Independence implies uncorrelation, but not the reverse. For Gaussian random variables X and Y, however, if they are uncorrelated, then they are also independent. ES150 Harvard SEAS 14

8 Conditional expectation The conditional expectation of X given Y is defined as For discrete r.v. s: For jointly continuous r.v. s: E [ ] X Y = y k = x i p X Y (x i y k ) E [ X Y = y ] = i x f X Y (x y)dx Similarly for the conditional expectation of a function of X, given Y g(x i )p X Y (x i y) for discrete X, Y E [g(x) Y = y] = i g(x) f X Y (x y)dx for jointly continuous X, Y The law of conditional expectation E[g(X)] = E Y [ E[g(X) Y ] ] ES150 Harvard SEAS 15 For discrete r.v. s E[g(X)] = k E [ g(x) y k ] py (y k ) For continuous r.v. s E[g(X)] = E [ g(x) y ] f Y (y)dy This law is very useful in calculating the expectation. Example: Defects in a chip. The total number of defects on a chip is X Poisson(α). The probability of finding an defect in the memory is p. Find the expected number of defects in the memory. ES150 Harvard SEAS 16

9 Two jointly Gaussian random variables The joint pdf of two jointly Gaussian r.v. s X and Y is { [ ]} 1 exp (x µx ) 2 (x µ 2(1 ρ f X,Y (x, y) = 2 XY ) 2ρ X )(y µ Y ) σx 2 XY σ X σ Y + (y µ Y ) 2 σy 2 2πσ X σ Y 1 ρ 2 XY The pdf is a function only of µ X, µ Y, σ X, σ Y, and ρ XY. If X and Y are jointly Gaussian then they are individually Gaussian; that is X N (µ X, σ 2 X ) and Y N (µ Y, σ 2 Y ). If X and Y are independent Gaussian random variables, then they are also jointly Gaussian with the above joint pdf (ρ XY = 0). In general, however, Gaussian random variables are not necessarily jointly Gaussian. +1 with probability 1 2 Example: Let X 1 N (0, 1) and X 2 = 1 with probability 1 2 be independent r.v. s. Let X 3 = X 1 X 2 then X 3 N (0, 1), but X 1 and X 3 are not jointly Gaussian. ES150 Harvard SEAS 17

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