# University of Arizona Normal Distributions Statistics Questions

SIE 430/530 : Engineering Statistics
( Lecture 13 )
Jian Liu
Department of Systems and Industrial Engineering
The University of Arizona
jianliu@email.arizona.edu, 520-621-6548(O)
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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List of Topics in Lecture 13
• Recall topics in lecture 12
• Order Statistics
• Principles of data reduction
– Sufficiency principle: sufficient statistics
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Recall Topics in Lecture 12
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Principles of Data Reduction
• Experimenters use the information in a
sample X1, …, Xn , denoted as X, to make
inferences about an unknown parameter q.
• Any statistic, T(X), defines a form of data
reduction or data summary.
• Data reduction in terms of a particular statistic
can be though of as a partition of the sample
space.
• Principles of data reduction:
– Sufficiency principle
– Likelihood principle
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Sufficient Principle
• A sufficient statistic for a parameter q is a
statistic that, in a certain sense, captures all the
information about q contained in the sample.
• Definition: A statistic T(X) is a sufficient statistic for q
if the conditional distribution of the sample X given
the value of T(X) does not depend on q.
• Theorem: if p(x|q) is the joint pdf or pmf of X and
q(T(x)|q) is the pdf or pmf of T(X), then T(X) is a
sufficient statistic for q if, for every x in the sample
space, the ratio p(x|q)/q(T(x)|q) is a constant as
function of q.
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Ex. 13.1: Binomial sufficient statistic
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Ex. 13.2: Normal sufficient statistic
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Another Way to Identify Sufficient Statistic
Factorization Theorem: Let f(x|q) denote the joint pdf
or pmf of a sample X. A statistic T(X) is a sufficient
statistic for q if and only if there exist function
g(T(x)|q) and h(x) such that, for all sample points x
and all parameter point q ,
f(x|q) = g(T(x)|q)∙h(x)
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Ex. 13.3: Normal sufficient statistic (another method)
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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SIE 430/530 : Engineering Statistics
( Lecture 12 )
Jian Liu
Department of Systems and Industrial Engineering
The University of Arizona
jianliu@email.arizona.edu, 520-621-6548(O)
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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List of Topics in Lecture 12
• Recall topics in lecture 11
• Chi-Squared RVs
• Derived distributions:
– t-Distribution
– F-Distribution
– Applications
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Recall Topics in Lecture 11
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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The Derived Distributions
• Student’s t-distribution
– Let X1, X2, …, Xn be a random sample of size n
from a N(µ, s2) distribution. The quantity
T=
X -µ
S/ n
has Student’s t-distribution with v=n-1 degrees of
freedom (i.e., T~tv) .
v +1
G(
)
1
1
2
fT (t | v) =
, -¥ < t < ¥ v (vp )1/ 2 (1 + t 2 / v)( v +1) / 2 G( ) 2 – T is the ratio of a N(0,1) RV and sqrt(independent c2 divided by its degrees of freedom, v) SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 4 4 Facts about Student’s t Distribution • E X = 0, for v>1
• VarX = v/(v-2) for v>2
• Related to F distribution (F1,v= t2v )
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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E X = 0, for v>1
VarX = v/(v-2) for v>2
Related to F distribution (F1,v= t2v )
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Ex. 12.1: the mean time it takes a crew to restart an aluminum
rolling mill after a failure is of interest. The crew was observed
over 25 occasions, and the results were X = 26.42 minutes and
variance S2 =12.28 minutes. If the repair time is normally
distributed, and the true mean (µ) is 30, what is the probability to
observe the X ≤ 26.42?
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Variance Ratio Distribution
• Ex. 12. 2: Let X1, X2, …, Xn be a random sample of
size n from a N(µX, sX2) population. Let Y1, Y2, …, Yn
be a random sample of size n from a N(µY, sY2)
population. How to compare the variability of the two
populations?
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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The Derived Distributions
• Snedecor’s F-distribution
– Let X1, X2, …, Xn be a random sample of size n
from a N(µX, sX2) population. Let Y1, Y2, …, Ym be a
random sample of size m from a N(µY, sY2)
population. The random variable
F = ( S X2 / s X2 ) /( SY2 / s Y2 )
has Snedecor’s F-distribution with v1=n-1 and
v2=m-1 degrees of freedom (i.e., F ~ Fv1 ,v2 ) .
v +v
G( 1 2 ) æ öv1 / 2
v1
x ( v1 – 2) / 2
2
f F ( x | v) =
, 0£ x 2
v2 – 2
v
(v + v – 2)
VarX = 2( 2 ) 2 1 2
, v2 > 4
v2 – 2 v1 (v2 – 4)
• As v1 and v2 increase, F tends to normal
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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• If X ~ Fv ,v , then 1/ X ~ Fv ,v
1
2
2
1
2
• If X ~ tq , then X ~ F1,q
• If X ~ Fv ,v , then
1
2
( v1 / v2 ) X
v v
~ beta ( 1 , 2 )
(1 + (v1 / v2 ) X )
2 2
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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• Ex. 12.3: If two random samples with size n=6 and
m=10 from two normal populations with equal
population variance, find the number b such that
S12
P( 2 £ b) = 0.95
S2
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Summary of Sampling Distribution
• For random sample from any distribution,
standardized sample mean converges to N(0,1) as
n increases (CLT).
• In normal case, standardized statistic (minus
sample mean with S, instead of s, as the
denominator) follows Student’s t(n-1).
• Sum of n squared unit normal variates follows
Chi-squared (n)
• In the normal case, sample variance has scaled
Chi-squared distribution.
• In the normal case, ratio of sample variances
from two different samples divided by their
respective d.f. has F distribution.
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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SIE 430/530 : Engineering Statistics
( Lecture 11 )
Jian Liu
Department of Systems and Industrial Engineering
The University of Arizona
jianliu@email.arizona.edu, 520-621-6548(O)
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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List of Topics in Lecture 11
• Recall topics in lecture 10
• Function of a Random Vector Random
samples
• Statistic: definition and properties
• Sample from normal distribution
• Statistics of exponential family
• Central Limit Theory
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Recall Topics in Lecture 10
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Statistics Starts from Today!
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Random Sample
• Definition: the random variables X1, X2, …, Xn are
called a random sample of size n from the population
f(x) if X1, X2, …, Xn are mutually independent random
variables and the marginal pdf or pmf of each Xi is the
same function f(x).
• Independently and identically distributed random
variables with pmf/pdf f(x) : iid sample
• Examples:
– Rolling a die n times
– Selecting 100 UA male students and measuring their heights.
• Note:
– sampling from a large population -> “nearly
independent”
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Joint pdf/pmf of Random Sample
• Given the iid random sample X1, X2, …, Xn,
n
f ( x1 , x2 ,…, xn ) = f ( x1 ) f ( x2 ) ! f ( xn ) = Õ f ( xi )
n
f ( x1 , x2 ,…, xn | θ) = Õ f ( xi | θ)
i =1
i =1

Ex. 11.1: let X1, X2, …, Xn be the time-to-failure of n identical
circuit boards randomly selected from a population, which has
an exponential distribution exp(b), what is the probability that
ALL the boards last more than 2 years.
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Statistic
• Definition: let X1, X2, …, Xn be a random
sample of size n from the population f(x) and
let T(x1, x2, …, xn ) be a real-valued or vectorvalued function whose domain includes the
sample space of (X1, X2, …, Xn ). Then the
random variable or random vector
Y=T(x1, x2, …, xn ) is called a statistic.
• The probability distribution of a statistic Y is
called the sampling distribution of Y.
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Sample Mean and Variance
• Sample Mean: denoted by Xn, is a statistic defined as
the arithmetic average of the values in a random
sample of size n.
• Sample Variance: denoted by Sn2, is a statistic
defined as:
• Remember: the observed value of the statistic is
denoted by lowercase letters. So, x , s2 and s denote
observed values of the RVs X, S2 , and S.
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Properties of Sample Statistics
• Let X1, X2, …, Xn be a random sample of size n from
the population f(x) with mean µ (finite) and variance
s2 (finite). Then
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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MGF of Sample Mean
• Theorem: Let X1, X2, …, Xn be a random sample from a
population with mgf MX(t). Then the mgf of the sample
mean is
M X (t ) = [ M X (t / n)]n
• Ex. 11.2 (Distribution of the mean) Let X1, X2, …, Xn be
a random sample from a N(µ, s2) population. Find the
distribution of sample mean.
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Statistics of Members of Exponential Family

Suppose X1, X2, …, Xn be a random sample from a population
with pdf/pmf a f(x|q), where
æ k
ö
f ( x | θ) = h( x)c(θ) exp ç å wi (θ)ti ( x) ÷
è i =1
ø
is a member of an exponential family. Define statistics T1, …, Tk
by
n
Ti ( X1 ,…, X n ) = å ti ( X j ), i = 1, 2,…, k
j =1
if the set {( w1 (q ), w2 (q ),…, wk (q )),q ÎQ}contains an open
subset of Rn, then the distribution of (T1,…, Tk) is an exponential
family of the form
k
æ
ö
n
fT (u1 ,…, uk | θ) = H (u1 ,…, uk ) [c(θ) ] exp ç å wi (θ)ui ÷
è i =1
ø
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Ex. 11.3: Sum of Bernoulli random variables
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Distribution of Sample Means
(Exponential Family)
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Distribution of Sample Means (General)
• Generally, the exact distribution is difficult to
calculate.
• What can be said about the distribution of the
sample mean when the sample is drawn from an
arbitrary population?
• In many cases we can approximate the
distribution of the sample mean when n is large
by a normal distribution.
• The famous Central Limit Theorem
SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O)
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Central Limit Theorem (CLT)

Let X1, …, Xn be independent and identically distributed (iid)
random variables with E(Xi)= µ (finite) and Var(Xi)= s2 (finite).
1 n
Define X n = å i =1 X i . Then, for any value x ∈ (−∞, ∞),
n
æ n ( Xn – µ)
ö
x 1
2
lim P ç
< x÷ = ò e- x /2 dx = F( x) n ®¥ ç ÷ -¥ 2 s è ø where Ф(x) is the standard normal distribution • In words, if Xis are normally distributed, the sample mean statistic will also be normally distributed. But with CLT, when n→∞, the function of the sample mean statistic, will be normally distributed regardless of the distribution of Xi’s. • In practice, CLT can be applied when n is sufficiently large. SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 15 15 Sampling from NORMAL Distribution SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 16 16 Properties of Sample Statistics from Normal Distribution • Let X1, X2, …, Xn be a random sample of size n from a N(µ, s2) distribution, and let X = (1/ n) å ( X - X ) Then dfdfds S = [1/(n - 1)] 2 n i =1 å X and n i =1 i 2 i SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 17 17 Ex. 11.4: The amount (unit: ounce) of fill dispensed by a bottle machine is normally distributed with mean at µ and s = 1.0. A sample of n=9 filled bottles is randomly selected, and the amount of fill are measured for each. (i) Find the probability that the sample mean will be within 0.3 ounce of the true mean, µ , for the chosen machine setting. (ii) How many observations should be randomly selected if we wish the sample mean to be within 0.3 ounce of the true mean, µ , with probability of 0.95? SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 18 18 Facts about Chi Squared RVs • We use the notation c p2 to denote a chi-squared random variable with p degree of freedom. 2 – If Z is a N(0,1) rv, the Z 2 ~ c1 , that is, the square of a standard normal rv is a chi squared rv. 2 – If X1, X2, …, Xn are independent and Xi ~ c pi , then X1 + X 2 + ! X n ~ c p21 + p2 +!+ pn SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 19 19 Ex. 11.5 (Ex. 11.4 Cont’d): assume we select a random sample of 10 bottles and measure the fill. If we calculate the sample variance S2 from these 10 bottles, how can we define the boundaries of an interval such that P(b1≤S2 ≤b2) = 0.90. SIE 430/530, Engineering Statistics, Jian Liu, the University of Arizona, jianliu@email.arizona.edu, 520-621-6548(O) 20 20

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