Expected Value and Variance

Transcription

1 MATH 38 Expected Value and Varance Dr. Neal, WKU We now shall dscuss how to fnd the average and standard devaton of a random varable X. Expected Value Defnton. The expected value (or average value, or mean µ ) of a dscrete random varable X s a weghted average gven by µ E[ X] P(X ). Note: If X has a fnte range { 1,,..., n }, then ts average value s E[ X ] 1 P( X 1 ) + P( X ) n P( X n ), whch s always a fnte sum. But f Range X s denumerable { 1,,..., n,... }, then E[ X ] s an nfnte sum and may or may not converge. Example 1. (a) Roll a far sx-sded de; record the value. What s the average value? (b) Roll a far sx-sded over and over untl you roll a 3. You wn $ n f you roll a 3 for the frst tme on the n th roll. What are your average earnngs? (c) Roll a sngle sx-sded de. If you roll a 1, then you get $5. If you roll a or 3, then you get $10. But f you roll a 4, 5, or, then you lose $0. Let X denote your change n fortune. Compute E[ X ]. Soluton. (a) Let X be the value of the de. Then Range X {1,, 3, 4, 5, }, and each value n the range occurs wth probablty 1/. Thus, E[ X ] P( X ) (b) Let X be your earnngs. Then Range X {,, 3,... }, and for n 1 we have P(X n ) 5 n 1 1 (from n 1 losses followed by the wn on the n th roll). Thus, E[ X ] P( X ) n P( X n ) n 1 n 5 n n 1. n 1 n 1 Although you would expect the game to end at some pont at whch you receve your payoff, the average payoff s nfnte.

2 (c) We have Range X { 0, 5, 10} wth the pdf of X gven by f ( 0) P( X 0) 3 f (5) P( X 5) 1 f (10) P(X 10). Thus, E[ X ] P( X ) $5.83. Condtonal Average Defnton. Let A be an event wth P(A) > 0. The condtonal average of a dscrete random varable X gven A s defned by E[ X A] P( X A) Range X 1 P( A) P ({X } A). P ( {X } A ) P(A) Theorem 1. (Law of Total Average) Suppose non-null events A 1, A,... form a partton of Ω. That s, the events are dsjont and ther unon s all of Ω. Then for any dscrete random varable X, we have E[ X ] E[ X A 1 ] P( A 1 ) + E[ X A ] P(A ) +... E[ X A ] P(A ). Proof. Usng propertes of seres and the probablty measure P, we have E[ X A ] P( A ) P( X A ) P( A ) P ( (X ) A ) P(A ) P(A ) P ((X ) A ) P ((X ) A ) rearrange the sum P (( X ) A ) P ( X ) A by dsjontness of partton P( X ) E[ X ].

3 Example. Roll one de. Let A 1 Roll a 1, A Roll a, 3 or 5, A 3 Roll a 4 or. Let X the value of the roll. What are E[ X A ]? Use these values to compute E[ X ]. Soluton. Frst, we have E[ X A 1 ] P( X A 1 ) 1 P( X 1 A 1 ) P( X 1 A 1 ) P( A 1 ) 1 P(A 1 ) P(A 1 ) 1. In other words, the average of the rolls that are 1 s 1. Next, E[ X A ] P( X A ) + 3P( X 3 A ) + 5P( X 5 A ) P(X A ) P( A ) P(X ) P( A ) P(X 3 A ) P( A ) P(X 3) P( A ) ( ) P( X 5) P(A ) + 5 P( X 5 A ) P(A ) 1 / 1 / / 1 / / 1 / That s, gven that you've rolled a, 3, or 5, the average roll s 10/3. Lastly, E[ X A 3 ] 4P( X 4 A 3 ) + P( X A 3 ) 4 P( X 4 A 3 ) P(A 3 ) 4 P( X 4) P( A 3 ) + (4 + ) P( X A 3 ) P(A 3 ) P(X ) P(A 3 ) 4 1 / 1 / / 1 / 3 Then, 3 E[ X ] E[ X A ] P(A ) 1P( A 1 ) P( A ) + 5P( A 3 ) Propertes of Expected Value We now prove some mportant and often used propertes the expected value for dscrete random varables. The proof of property () uses the precedng concept of condtonal expectaton.

4 Theorem. () The expected value of a constant s that constant: E[c ] c. () Expected value s a lnear operator on dscrete random varables; that s, constants factor out of an expected value and the expected value of a sum s the sum of the expected values: E[c X ] c E[ X ] and E[ X + Y ] E[ X] + E[Y]. () If X 0, then E[ X ] 0. In partcular, E[ Z ] 0 for any random varable Z. Proof. () Let X c wth probablty 1. Then E[c ] c P( X c) c 1 c. () If c 0, then E[c X ] E[0] 0 0 E[ X]. Otherwse, f X has range wth dstnct values { 1,,... }, then c X has range wth dstnct values {c 1, c,... } and P(X c ) P(X ). Hence, E[c X ] c P( X c ) c P( X ) c E[ X ]. Next, let X have range wth dstnct values { 1,,... } and let Y have range wth dstnct values {l 1, l,... }. Then the range of X +Y s the collecton of possble sums { 1 + l 1, 1 + l,..., + l 1, + l,..., j + l 1, j + l,... } (wth some of these values possbly repeatng). Then E[ X +Y ] P( X + Y ) ( + l j ) P( X,Y l j ) P( X, Y l j ) + l j P( X,Y l j ) ( ) P( X,Y l j ) + l j P( X, Y l j ) P( X, Y l j ) + l j P( X ) P(Y l j X ) j P {X Y l j } + P( X ) l j P(Y l j X ) P( X ) + P( X )E[Y X ] E[ X] + E[Y].

5 () Assume that every value n the range of X s non-negatve. Because P(X ) also s non-negatve for every, we have E[ X ] P( X ) 0. Example 3. Fnd the average sum when rollng two dce. Soluton. (Hard way) Let X 1 be the value of one de and let X be the value of the other de. The sum X 1 + X has range {, 3, 4,..., 1}. Then, 1 E[ X 1 + X ] P( X 1 + X ) (Easer) By lnearty we have E[ X 1 + X ] E[X 1 ] + E[ X ] Varance and Standard Devaton Defnton. Let X be a random varable wth µ E[ X] beng fnte. The varance of X s then defned by σ Var( X ) E [(X E[ X]) ] E [( X µ ) ] ( µ ) P( X ). Because the random varable ( X E[ X ]) s non-negatve, ts expected value s nonnegatve. That s, Var( X ) 0. Thus, we can defne the standard devaton of X by σ Var(X). The formal defnton of varance s rarely used n practce. When computng the varance of a dscrete random varable, t s more common to use the followng result: ( ). Theorem 3. Let X be a dscrete random varable. Then Var( X) E[X ] E[ X]

6 Proof. Usng µ E[X] and the propertes of expected value we have Var( X) E [( X µ ) ] E[ X µ X + µ ] E[X ] µ E[ X ] + E[µ ] E[X ] µ + µ E[X ] µ E[X ] (E[ X]). In order to apply ths result, we frst compute E[ X ] P(X ), then compute E[ X ] by E[ X ] P(X ). Then Var( X) E[X ] ( E[ X] ) and σ Var( X). Example 4. Roll a sngle sx-sded de. If you roll a 1, then you get $5. If you roll a or 3, then you get $10. But f you roll a 4, 5, or, then you lose $0. Let X denote your change n fortune. Compute the standard devaton of X. Soluton. As n Ex. 1(c), we have P(X 0) 3, P(X 5) 1, P(X 10) and E[ X ] Then E[ X ] ( 0) Thus, Var( X ) E[X ] ( E[ X ]) , and σ Here we have 0 occurrng half the tme, 5 occurrng one-sxth of the tme, and 10 occurrng one-thrd of the tme. The average value s then about The standard devaton of 14.4 gves a way of measurng the average spread from the mean. The actual values n the range, 0, 5, 10, all dffer from the sngle average value of And n ths case, the average spread from 5.83 s about Example 5. Let m and n be non-negatve ntegers wth m n. Let X be a randomly chosen nteger from m to n. Fnd the mean and varance of X. Soluton. The range of X s the set of ntegers {m,..., n } that has n m +1 elements. Snce X s chosen randomly, each value n the range occurs wth the equal probablty of 1 / (n m +1). Thus, the expected value of X s

7 n E[X] m 1 n m n m n m n m + 1 m + n The varance of X s gven by Var( X) E[X ] ( E[ X] ) n P( X ) m 1 n m n m +1 n(n + 1) 1 n m 1 n m (m 1)m n m + n + m (n + m)(n m) + n + m (n + m)(n m +1) m + n n(n +1)(n + 1) (n m)(n m + ). 1 1 n m +1 n m (m 1) m(m 1) m + n m + n Independent Random Varables Intutvely, we thn of random varables X and Y as beng ndependent when ther values are not affected by each other. That s, the fact that X a has no bearng on whether Y b, and vce versa. We formally defne ndependence as follows: Defnton. Random varables X 1, X,..., X n are ndependent f and only f ( ) P X j a j P {X 1 a 1 } {X a }... {X a } for all possble subsequences X 1,..., X. j1 ( ),

8 Example. Flp a con 3 tmes. Let X count the number of Heads on the th flp. Then X s ether 1 or 0 and the X are ndependent, and the probablty of gettng 3 Heads n a row s P(X 1 1 X 1 X 3 1) P( X 1 1) P(X 1) P(X 3 1) Note: For convenence, we often wrte P(X a X j b) as P(X a, X j b), and we denote that X and Y are ndependent by X Y. Theorem 4. Let X and Y be ndependent. Then E[ X Y] E[ X] E[Y]. Proof. Let X have range wth dstnct values { 1,,... } and let Y have range wth dstnct values {l 1, l,... }. Then the range of X Y s the collecton of possble products { 1 l 1, 1 l,..., l 1, l,..., j l 1, j l,... } Then, E[ X Y] P(X Y ) l j P(X, Y l j ) l j P( X ) P(Y l j ) P( X ) l j P(Y l j ) by ndep. P( X ) l j P(Y l j ) j ( P( X ) E[Y] ) E[Y] P( X ) E[Y] E[ X] E[ X ] E[Y ]. Note: By nducton, the prevous result easly extends to a fnte sequence. That s, f random varables X 1, X,..., X n are ndependent, then E[ X 1 X... X n ] E[ X 1 ] E[ X ]... E[X n ]. Example 7. (a) Roll two dce. What s the average product of the two values? (b) Roll one de. Square the result. What s the average square? How does t compare wth the square of the average roll?

9 Soluton. (a) (Hard Way) Let X 1 be the value of one de and let X be the value of the other de. Then both have range {1,,..., }. So E[ X 1 X ] P(X Y ) l P( X,Y l ) 1 j (Easer) By ndependence we have E[ X 1 X ] E[X 1 ] E[ X ] (b) Now let X be the value on the rolled de. Then X has range 1, 4, 9, 1, 5, 3. So the average square s E[ X ] But E[ X ] (3. 5) Thus, E[ X ] E[ X ]. Notes: () Because X s not ndependent of tself, we cannot say that E[ X ] E[ X X] E[ X ] E[X ]. () Because E[ X ] E[ X ] Var(X ) E[( X µ) ] 0, we always have E[ X ] E[ X]. () If X s the value on a rolled de, then Var( X) E[ X ] E[ X] 91 and σ 35 / Other Facts About Varance Let X and Y be a random varables and let a and b be constants. Then () Var(a) 0 () Var(a X) a Var( X) () Var( X + b) Var(X ) (v) Var(a X + b) a Var(X ) (v) If X and Y are ndependent, then Var( X + Y) Var(X ) + Var(Y).

10 Proofs. () Var(a) E[a ] ( E[a]) a (a) 0. () Var(a X ) E[(aX ) ] (E[aX ]) E[a X ] (a E[X ]) a E[ X ] a ( E[ X ]) ( ) a E[ X ] ( E[X ]) a Var( X ). (v) Smply combne Propertes () and (). (v) E[( X + Y) ] E[ X + XY + Y ] E[ X ] + E[ XY] + E[Y ] E[ X ] + E[ X ]E[Y] + E[Y ] by ndependence Hence, Var( X + Y) E[(X + Y) ] E[ X + Y] [ ] [ ] [ ] ( ) Var( X + b) E (( X + b) E[ X + b] ) E ( X + b E[ X] b) E (( X E[X ]) Var( X). E[ X ] + E[X ]E[Y] + E[Y ] ( E[ X ] + E[Y] ) ( ) E[ X ] + E[X ]E[Y] + E[Y ] E[ X ] + E[ X ]E[Y] + E[Y] E[ X ] E[ X] + E[Y ] E[Y] Var(X ) + Var(Y). Note: If X and Y are ndependent, then Var( X Y) Var( X ) + Var(Y) by Propertes (v) and (). By nducton, Property (v) extends to a fnte sequence. That s, f random varables X 1, X,..., X n are ndependent, then Var( X 1 + X X n ) Var( X 1 ) + Var(X ) Var(X n ).

11 Exercses 1. Draw one card from a standard dec. If you draw a number card ( through 10), then X wll be 10. If you draw a face card, then X wll be 0. If you draw an Ace, then X wll be 30. (a) Compute the mean and standard devaton of X. (b) Compute the condtonal average of X gven that you draw a number card or a face card.. You mae a bet for whch you wn wth probablty p and lose wth probablty q 1 p, where 0 < p < 1. If you wn, you gan $a and f you lose then you drop $b. Let X be your change n fortune so that X s ether +a or b. (a) Derve E[ X ]. (b) The casno wants E[ X ] 0. Use your formula for E[ X ] from (a) to solve for what the payoff a should be to mantan the nequalty E[ X ] Roll two far sx-sded dce and let (ω 1, ω ) denote the resultng par. Defne a random varable X by X ((ω 1, ω )) ω 1 ω. Compute the mean and standard devaton of X. 4. The Fbonacc sequence {F n } s defned by F 1 1, F 1, and F n F n 1 + F n for n 3. Let S be sum of the frst sx Fbonacc numbers. A based sx-sded de s such that the probablty of rollng the value s F / S for 1,,...,. Roll ths de and let X be the value. Compute the mean and standard devaton of X.