STAT 111 Recitation 4

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1 STAT 111 Recitation 4 Linjun Zhang September 29, 2017

2 Misc. Mid-term exam time: 6-8 pm, Wednesday, Oct. 11 The mid-term break is Oct. 5-8 The next recitation class sis Oct. 1 1

3 Recap of the concepts Long formula for the mean and variance of X. 2

4 Recap of the concepts Long formula for the mean and variance of X. Suppose that X satisfies binomials distribution Binomial(n, θ), then the mean of X is nθ, and the variance of X is nθ(1 θ). 2

5 Recap of the concepts Long formula for the mean and variance of X. Suppose that X satisfies binomials distribution Binomial(n, θ), then the mean of X is nθ, and the variance of X is nθ(1 θ). The mean and variance of T n and X (T n denotes the sum and X denotes the average) 2

6 Recap of the concepts Long formula for the mean and variance of X. Suppose that X satisfies binomials distribution Binomial(n, θ), then the mean of X is nθ, and the variance of X is nθ(1 θ). The mean and variance of T n and X (T n denotes the sum and X denotes the average) The mean and variance of D (D = X 1 X 2 denotes the difference between X 1 and X 2 ) 2

7 Mean and Variance of T n and X Given n i.i.d. random variables X 1, X 2,..., X n with mean µ and variance σ 2, we define two other random variables: The sum, denoted by T n : T n = X 1 + X X n ; The average, denoted by X, X = X1+X2+...+Xn n = Tn n. Must know formula for T n and X Mean of T n = nµ; Variance of T n = nσ 2 ; Mean of X = µ; Variance of X = σ 2 /n.

8 Difference of random variables Suppose there are two independent random variables X 1 with mean µ 1 and variance σ1 2, and X 2 with mean µ 2 and variance σ2 2. The difference D is defined by D = X 1 X 2. Must know formula for D Mean of D = µ 1 µ 2 ; Variance of D = σ σ2 2. 4

9 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X 5

10 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X 5

11 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? 5

12 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 5

13 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 5

14 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 5

15 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 Variance of T 10? 5

16 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 Variance of T 10? = 100 5

17 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 Variance of T 10? = 100 Mean of X 1 X 2? 5

18 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 Variance of T 10? = 100 Mean of X 1 X 2? 0 5

19 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 Variance of T 10? = 100 Mean of X 1 X 2? 0 Variance of X 1 X 2? 5

20 Practice problems If X 1,..., X 10 are i.i.d. random variables with mean and variance 10 What s the mean of X Variance of X =? = 1 Mean of T 10? 0 Variance of T 10? = 100 Mean of X 1 X 2? 0 Variance of X 1 X 2? = 20 5

21 Roadmap Continuous random variables and normal distribution Z-chart (VERY IMPORTANT!!) 6

22 Continuous random variables A random variable X is continuous if it can take any value in some continuous range of values, denoted by (L, H). Every continuous random variable X has an associated density function f (x). 7

23 Continuous random variables A random variable X is continuous if it can take any value in some continuous range of values, denoted by (L, H). Every continuous random variable X has an associated density function f (x). The probability that the continuous random variable takes a value in the range (a, b) is 7

24 Continuous random variables From a calculus point of view this probability is obtained by integrating this density function over the range a to b. Prob(a < X < b) = b a f (x) dx. As a particular case, H L f (x) dx = 1. 8

25 The mean and variance of a continuous random variable The mean µ and variance σ 2 of a continuous random variable X having range (L, H) and density function f (x) are defined respectively by µ = H L xf (x) dx and σ 2 = H L (x µ) 2 f (x) dx = H L x 2 f (x) dx µ 2. 9

26 Normal distribution The continuous random variable X has a normal distribution with mean µ and variance σ 2 (aka, N(µ, σ 2 )), then the density function f (x) is given by the formula f (x) = 1 2πσ e (x µ)2 2σ 2, < x <. 10

27 Normal distribution The continuous random variable X has a normal distribution with mean µ and variance σ 2 (aka, N(µ, σ 2 )), then the density function f (x) is given by the formula f (x) = 1 2πσ e (x µ)2 2σ 2, < x <. If a random variable has distribution N(0, 1), then it is called a standardized normal distribution, denoted by Z. 10

28 Normal distribution The continuous random variable X has a normal distribution with mean µ and variance σ 2 (aka, N(µ, σ 2 )), then the density function f (x) is given by the formula f (x) = 1 2πσ e (x µ)2 2σ 2, < x <. If a random variable has distribution N(0, 1), then it is called a standardized normal distribution, denoted by Z. If a random variable X has distribution N(µ, σ 2 ),then Z = X µ has standardized normal σ distribution. 10

29 Normal distribution The continuous random variable X has a normal distribution with mean µ and variance σ 2 (aka, N(µ, σ 2 )), then the density function f (x) is given by the formula f (x) = 1 2πσ e (x µ)2 2σ 2, < x <. If a random variable has distribution N(0, 1), then it is called a standardized normal distribution, denoted by Z. If a random variable X has distribution N(µ, σ 2 ),then Z = X µ has standardized normal σ distribution. Bell-shaped curve 10

30 Z-chart The Z-chart is designed for a standard normal random variable, i.e. N(0, 1). It gives less than probabilities for positive values of z, i.e. Prob(Z a) when a > 0. 11

31 Z-chart Z has normal distribution with mean 0 and variance 1 Prob(Z a) when a 0; Practice: Prob(Z 0.24), Prob(Z 0.51) 12

32 Z-chart Z has normal distribution with mean 0 and variance 1 Prob(Z a) when a 0; Practice: Find b, such that Prob(Z b) =

33 Z-chart Z has normal distribution with mean 0 and variance 1 Prob(Z a) when a < 0 Key fact: Prob(Z a) = 1 Prob(Z a). Practice: Prob(Z 0.24), Prob(Z 0.49) 12

34 Z-chart Z has normal distribution with mean 0 and variance 1 Prob(a < Z b) when a < b. Key fact: Prob(a < Z b) = Prob(Z b) Prob(Z a) Practice: Prob(0.24 < Z 0.51), Prob( 0.24 < Z 0.51) 12

35 Z-chart X has normal distribution with mean µ and variance σ 2. Prob(X a), Prob(a < X b). Key fact: X µ σ has distribution N(0, 1). Practice : Prob( < X 4) when X satisfies N(2, 16). 1

36 Z-chart X has normal distribution with mean µ and variance σ 2. Prob(X a), Prob(a < X b). Key fact: X µ σ has distribution N(0, 1). Practice : Find b, such that Prob(X b) = when X satisfies N(2, 16). 1

37 Z-chart One more: Prob(7 < X 10) when X has distribution N(8, 9). Z-chart 14

38 Z-chart One more: Prob(7 < X 10) when X has distribution N(8, 9). Solutions 1 Let Z = X 8 9 = X 8. 14

39 Z-chart One more: Prob(7 < X 10) when X has distribution N(8, 9). Solutions 1 Let Z = X 8 9 = X 8. 2 Prob(7 < X 10) = Prob( 7 8 < X ) = Prob( 0. < Z 0.67) 14

40 Z-chart One more: Prob(7 < X 10) when X has distribution N(8, 9). Solutions 1 Let Z = X 8 9 = X 8. 2 Prob(7 < X 10) = Prob( 7 8 < X ) = Prob( 0. < Z 0.67) Prob( 0. < Z 0.67) = Prob(Z 0.67) Prob(Z 0.) 14

41 Z-chart One more: Prob(7 < X 10) when X has distribution N(8, 9). Solutions 1 Let Z = X 8 9 = X 8. 2 Prob(7 < X 10) = Prob( 7 8 < X ) = Prob( 0. < Z 0.67) Prob( 0. < Z 0.67) = Prob(Z 0.67) Prob(Z 0.) 4 Prob(Z 0.67) (1 Prob(Z 0.)) = ( ) =

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