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Discrete Random Variables Randomness Discrete Random Variables Randomness • The word random effectively means unpredictable • In engineering practice we may treat some signals as random to simplify the analysis even though they may not actually be random Random Variable Defined X A random variable () is the assignment of numerical values to the outcomes of experiments Random Variables Examples of assignments of numbers to the outcomes of experiments. Discrete-Value vs Continuous- Value Random Variables •A discrete-value (DV) random variable has a set of distinct values separated by values that cannot occur • A random variable associated with the outcomes of coin flips, card draws, dice tosses, etc... would be DV random variable •A continuous-value (CV) random variable may take on any value in a continuum of values which may be finite or infinite in size Probability Mass Functions The probability mass function (pmf ) for a discrete random variable X is P x = P X = x . X () Probability Mass Functions A DV random variable X is a Bernoulli random variable if it takes on only two values 0 and 1 and its pmf is 1 p , x = 0 P x p , x 1 X ()= = 0 , otherwise and 0 < p < 1. Probability Mass Functions Example of a Bernoulli pmf Probability Mass Functions If we perform n trials of an experiment whose outcome is Bernoulli distributed and if X represents the total number of 1’s that occur in those n trials, then X is said to be a Binomial random variable and its pmf is n x nx p 1 p , x 0,1,2,,n P x () {} X ()= x 0 , otherwise Probability Mass Functions Binomial pmf Probability Mass Functions If we perform Bernoulli trials until a 1 (success) occurs and the probability of a 1 on any single trial is p, the probability that the k1 first success will occur on the kth trial is p()1 p . A DV random variable X is said to be a Geometric random variable if its pmf is x1 p 1 p , x 1,2,3,... P x () {} X ()= 0 , otherwise Probability Mass Functions Geometric pmf Probability Mass Functions If we perform Bernoulli trials until the rth 1 occurs and the probability of a 1 on any single trial is p, the probability that the rth success will occur on the kth trial is k 1 r kr P()rth success on kth trial = p ()1 p . r 1 A DV random variable Y is said to be a negative - Binomial or Pascal random variable with parameters r and p if its pmf is y 1 r yr p 1 p , y r,r +1,, P y () {} Y ()= r 1 0 , otherwise Probability Mass Functions Negative Binomial (Pascal) pmf Probability Mass Functions Suppose we randomly place n points in the time interval 0 t < T with each point being equally likely to fall anywhere in that range. The probability that k of them fall inside an interval of length t < T inside that range is n k nk n! k nk P k inside t = p ()1 p = p ()1 p k k!()n k ! where p = t / T is the probability that any single point falls within t . Further, suppose that as n , n / T = , a constant. If is constant and n that implies that T and p 0. Then is the average number of points per unit time, over all time. Probability Mass Functions Events occurring at random times Probability Mass Functions It can be shown that n k k P k inside t = lim 1 = e k! n n k! =e where = t. A DV random variable is a Poisson random variable with parameter if its pmf is x e , x 0,1,2,, P x {} X ()= x! 0 , otherwise Cumulative Distribution Functions The cumulative distribution function (CDF) is defined by F x P X x . X ()= For example, the CDF for tossing a single die is u()x 1 + u()x 2 + u()x 3 F x 1/6 X ()= () u x 4 u x 5 u x 6 + () + () + () 1 , x 0 where u()x 0 , x < 0 Functions of a Random Variable Consider a transformation from a DV random variable X to another DV random variable Y through Y = g()X . If the 1 function g is invertible, then X = g ()Y and the pmf for Y is P y = P g1 y where P x is the pmf for X. Y () X ()() X () Functions of a Random Variable If the function g is not invertible the pmf and pdf of Y can be found by finding the probability of each value of Y. Each value of X with non-zero probability causes a non-zero probability for the corresponding value of Y. So, for the ith value of Y, P Y = y = P X = x + P X = x + i i,1 i,2 n + P X = x = P X = x i,n i,k k=1 The function to the right is an example of a non-invertible function. Expectation and Moments Imagine an experiment with M possible distinct outcomes performed N times. The average of those N outcomes is 1 M X n x where x is the ith distinct value of X and n = i i i i N i=1 is the number of times that value occurred. Then M M M 1 ni X n x x r x = i i = i = i i N i=1 i=1 N i=1 The expected value of X is M M M ni E X = lim xi = lim ri xi = P X = xi xi N N i=1 N i=1 i=1 Expectation and Moments Three common measures are used in statistics to indicate an "average" of a random variable are the mean, the mode and the median. The mean is the sum of the values 1 M divided by the number of values X n x . = i i N i=1 The mode is the value that occurs most often. P x P x for all x. X ()mode X () The median is the value for which an equal number of values fall above and below. P X > x = P X < x X ()median X ()median Expectation and Moments The first moment of a random variable is its expected value M E X x P X x = i = i i=1 The second moment of a random variable is its mean-squared value (which is the mean of its square, not the square of its mean). M E X 2 = x2 P X = x i i i=1 The name "moment" comes from the fact that it is mathematically the same as a moment in classical mechanics. Expectation and Moments The nth moment of a random variable is defined by M E X n = xn P X = x i i i=1 The expected value of a function g of a random variable is M E g X = g X P X = x () () i i=1 Expectation and Moments A central moment of a random variable is the moment of that random variable after its expected value is subtracted. n M n E X E X = x E X P X=x () ()i i i=1 The first central moment is always zero. The second central moment (for real-valued random variables) is the variance, 2 M 2 2 = E X E X = x E X P X=x X () ()i i i=1 The variance of X can also be written as Var X . The positive square root of the variance is the standard deviation. Expectation and Moments Properties of expectation E a a ,EaX a E X ,E X E X = = n = n n n where a is a constant. These properties can be use to prove the handy relationship, 2 = E X 2 E2 X X The variance of a random variable is the mean of its square minus the square of its mean. Another handy relation is Var aX b a2 Var X . + = Conditional Probability Mass Functions The concept of conditional probability can be extended to a conditional probability mass function defined by P x X () , x A P x = P A X |A () 0 , otherwise where A is the condition that affects the probability of X. Similarly the conditional expected value of X is E X | A x P x and the conditional cumulative = X |A () xB distribution function for X is F x = P X x | A . X |A () Conditional Probability Let A be A = {}X a where a is a constant. P ()X x ()X a Then F x P X x | X a . X |A ()= = P X a If a x then P X x X a P X a and () () = P X a F x P X x | X a 1. X |A ()= = = P X a If a x then P X x X a P X x and () () = P X x F x F x P X x | X a X () X |A ()= = = P X a F a X ().
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