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851_lectures17_24

# 851_lectures17_24 - Statistics 851(Fall 2013 Prof Michael...

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Statistics 851 (Fall 2013) October 16, 2013 Prof. Michael Kozdron Lecture #17: Expectation of a Simple Random Variable Recall that a simple random variable is one that takes on ±nitely many values. DeFnition. Let (Ω , F , P )beaprobab i l ityspace . Arandomvar iab le X :Ω R is called simple if it can be written as X = n ° i =1 a i 1 A i where a i R , A i ∈F for i =1 , 2 ,...,n .W ede±netheexpe c ta t iono f X to be E ( X )= n ° i =1 a i P { A i } . Example 17.1. Consider the probability space (Ω , B 1 , P )whereΩ=[0 , 1], B 1 denotes the Borel sets of [0 , 1], and P is the uniform probability on Ω. Suppose that the random variable X R is de±ned by X ( ω 4 ° i =1 a i 1 A i ( ω ) where a 1 =4 , a 2 =2 , a 3 , a 4 = 1, and A 1 =[0 , 1 2 ) ,A 2 =[ 1 4 , 3 4 ) 3 =( 1 2 , 7 8 ] 4 7 8 , 1] . Show that there exist ±nitely many real constants c 1 ,...,c n and disjoint sets C 1 ,...,C n ∈B 1 such that X = n ° i =1 c i 1 C i . Solution. We ±nd X ( ω 4 , if 0 ω< 1 / 4 , 6 , if 1 / 4 1 / 2 , 2 , if ω / 2 , 3 , if 1 / 2 <ω< 3 / 4 , 1 , if 3 / 4 7 / 8 , 0 , if ω =7 / 8 , 1 , if 7 / 8 1 , so that X = 7 ° i =1 c i 1 C i where c 1 , c 2 =6 , c 3 , c 4 =3 , c 5 , c 6 =0 , c 7 = 1and C 1 , 1 4 ) ,C 2 1 4 , 1 2 ) 3 = { 1 2 } 4 1 2 , 3 4 ) 5 3 4 , 7 8 ) 6 = { 7 8 } 7 7 8 , 1] . 17–1

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Proposition 17.2. If X and Y are simple random variables, then E ( αX + βY )= α E ( X )+ β E ( Y ) for every α , β R . Proof. Suppose that X and Y are simple random variables with X = n ° i =1 a i 1 A i and Y = m ° j =1 b j 1 B j where A 1 ,...,A n ∈F and B 1 ,...,B m each partition Ω. Since αX = α n ° i =1 a i 1 A i = n ° i =1 ( αa i )1 A i we conclude by defnition that E ( αX n ° i =1 ( αa i ) P { A i } = α n ° i =1 a i P { A i } = α E ( X ) . The prooF oF the theorem will be completed by showing E ( X + Y E ( X E ( Y ). Notice that { A i B j :1 i n, 1 j m } consists oF pairwise disjoint events whose union is Ω and X + Y = n ° i =1 m ° j =1 ( a i + b j )1 A i B j . ThereFore, by defnition, E ( X + Y n ° i =1 m ° j =1 ( a i + b j ) P { A i B j } = n ° i =1 m ° j =1 a i P { A i B j } + n ° i =1 m ° j =1 b j P { A i B j } = n ° i =1 a i P { A i } + m ° j =1 b j P { B j } and the prooF is complete. Fact. IF X and Y are simple random variables with X Y ,then E ( X ) E ( Y ) . Exercise 17.3. Prove the previous Fact. 17–2
Having already defned E ( X )Forsimplerandomvariables,ourgoalnowistoconstruct E ( X ) in general. To that end, suppose that X is a positive random variable .T h a ti s , X ( ω ) 0 For all ω Ω. (We will need to allow X ( ω ) [0 , + ]Forsomecons istency .) Defnition. IF X is a positive random variable, defne the expectation oF X to be E ( X ) = sup { E ( Y ): Y is simple and 0 Y X } . That is, we approximate positive random variables by simple random variables. OF course, this leads to the question oF whether or not this is possible. Fact. ±or every random variable X 0, there exists a sequence ( X n )o Fpo s i t ive ,s imp le random variables with X n X (that is, X n increases to X ).

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851_lectures17_24 - Statistics 851(Fall 2013 Prof Michael...

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