Lecture 07 - 1 Discrete Random Variables We will now review...

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1 Discrete Random Variables We will now review a number of well known discrete random variables. In each case we will begin with an example. We will then give the definition, often through the specification of the probability mass function, and will compute the mean and the variance. 1.1 Multinomial Consider a partition of the sample space S into k mutually exclusive and collectively exhaustive subsets A 1 ,...,A k and let p j = P ( A j ) 0 and k j =1 p j = 1. Suppose we repeat the experiment n times under identical conditions. Then we may be interested in the probability that A j occurs x j times for j = 1 ,...,k with k j =1 x j = n . Example: Suppose that 10 students are each to independently select one of three sections of a given course. Suppose that each student has probability 1/3 of selecting Section 1, 1/6 of selecting Section 2, and 1/2 of selecting Section 3. What is the probability that x 1 = 3 students select Section 1, x 2 = 2 students select Section 2, and x 3 = 5 students select Section 3? Consider the following ordering of the selection: (1 , 1 , 1 , 2 , 2 , 3 , 3 , 3 , 3 , 3). In this ordering the first three students select Section 1, the following two students select Section 2 and the last five select Section 3. The probability of this selection is (1 / 3) 3 * (1 / 6) 2 * (1 / 2) 5 , but there are altogether ( 10 3 , 2 , 5 ) = 2520 arrangements that result in the desired number of students in each section. Thus, the probability is given by p (3 , 2 , 5) = ± 10 3 , 2 , 5 (1 / 3) 3 * (1 / 6) 2 * (1 / 2) 5 = 0 . 040509259 . In general, if the sample space is partitioned into
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Lecture 07 - 1 Discrete Random Variables We will now review...

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