Bbs11 ppt ch13

Basic Business Statistics, 11e © 2009 Prentice-Hall, Inc. Chap 13-1
Chapter 13
Simple Linear Regression
Basic Business Statistics
11th Edition

Basic Business Statistics, 11e © 2009 Prentice-Hall, Inc.. Chap 13-2
Learning Objectives
In this chapter, you learn:
 How to use regression analysis to predict the value of
a dependent variable based on an independent
variable
 The meaning of the regression coefficients b0 and b1
 How to evaluate the assumptions of regression
analysis and know what to do if the assumptions are
violated
 To make inferences about the slope and correlation
coefficient
 To estimate mean values and predict individual values

Correlation vs. Regression
 A scatter plot can be used to show the
relationship between two variables
 Correlation analysis is used to measure the
strength of the association (linear relationship)
between two variables
 Correlation is only concerned with strength of the
relationship
 No causal effect is implied with correlation
 Scatter plots were first presented in Ch. 2
 Correlation was first presented in Ch. 3

Introduction to
Regression Analysis
 Regression analysis is used to:
 Predict the value of a dependent variable based on
the value of at least one independent variable
 Explain the impact of changes in an independent
variable on the dependent variable
Dependent variable: the variable we wish to
predict or explain
Independent variable: the variable used to predict
or explain the dependent
variable

Model
 Only one independent variable, X
 Relationship between X and Y is
described by a linear function
 Changes in Y are assumed to be related
to changes in X

Types of Relationships
Y
X
Y
X
Y
Y
X
X
Linear relationships Curvilinear relationships

Y
X
Y
X
Y
Y
X
X
Strong relationships Weak relationships
(continued)

Y
X
Y
X
No relationship
(continued)

ii10i εXββY 
Linear component
Model
Population
Y intercept
Population
Slope
Coefficient
Random
Error
term
Dependent
Variable
Independent
Variable
Random Error
component

(continued)
Random Error
for this Xi value
Y
X
Observed Value
of Y for Xi
Predicted Value
of Y for Xi
ii10i εXββY 
Xi
Slope = β1
Intercept = β0
εi
Model

i10i XbbYˆ 
The simple linear regression equation provides an
estimate of the population regression line
Equation (Prediction Line)
Estimate of
the regression
intercept
Estimate of the
regression slope
Estimated
(or predicted)
Y value for
observation i
Value of X for
observation i

The Least Squares Method
b0 and b1 are obtained by finding the values of
that minimize the sum of the squared
differences between Y and :
2
i10i
2
ii ))Xb(b(Ymin)Yˆ(Ymin  
Yˆ

Finding the Least Squares
Equation
 The coefficients b0 and b1 , and other
regression results in this chapter, will be
found using Excel or Minitab
Formulas are shown in the text for those
who are interested

 b0 is the estimated average value of Y
when the value of X is zero
 b1 is the estimated change in the
average value of Y as a result of a
one-unit change in X
Interpretation of the
Slope and the Intercept

Example
 A real estate agent wishes to examine the
relationship between the selling price of a home
and its size (measured in square feet)
 A random sample of 10 houses is selected
 Dependent variable (Y) = house price in $1000s
 Independent variable (X) = square feet

Example: Data
House Price in $1000s
(Y)
Square Feet
(X)
245 1400
312 1600
279 1700
308 1875
199 1100
219 1550
405 2350
324 2450
319 1425
255 1700

0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Square Feet
HousePrice($1000s)
Example: Scatter Plot
House price model: Scatter Plot

Example: Using Excel

Example: Excel Output
Regression Statistics
Multiple R 0.76211
R Square 0.58082
Adjusted R Square 0.52842
Standard Error 41.33032
Observations 10
ANOVA
df SS MS F Significance F
Regression 1 18934.9348 18934.9348 11.0848 0.01039
Residual 8 13665.5652 1708.1957
Total 9 32600.5000
Coefficients Standard Error t Stat P-value Lower 95% Upper 95%
Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386
Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580
The regression equation is:
feet)(square0.1097798.24833pricehouse 

Example: Minitab Output
The regression equation is
Price = 98.2 + 0.110 Square Feet
Predictor Coef SE Coef T P
Constant 98.25 58.03 1.69 0.129
Square Feet 0.10977 0.03297 3.33 0.010
S = 41.3303 R-Sq = 58.1% R-Sq(adj) = 52.8%
Analysis of Variance
Source DF SS MS F P
Regression 1 18935 18935 11.08 0.010
Residual Error 8 13666 1708
Total 9 32600
The regression
equation is:
house price = 98.24833 +
0.10977 (square feet)

0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Square Feet
HousePrice($1000s)
Simple Linear Regression Example:
Graphical Representation
House price model: Scatter Plot and Prediction Line
Slope
= 0.10977
Intercept
= 98.248

Example: Interpretation of bo
 b0 is the estimated average value of Y when the
value of X is zero (if X = 0 is in the range of
observed X values)
 Because a house cannot have a square footage
of 0, b0 has no practical application

Example: Interpreting b1
 b1 estimates the change in the average
value of Y as a result of a one-unit
increase in X
 Here, b1 = 0.10977 tells us that the mean value of a
house increases by .10977($1000) = $109.77, on
average, for each additional one square foot of size

317.85
0)0.1098(20098.25
(sq.ft.)0.109898.25pricehouse



Predict the price for a house
with 2000 square feet:
The predicted price for a house with 2000
square feet is 317.85($1,000s) = $317,850
Example: Making Predictions

0
50
100
150
200
250
300
350
400
450
0 500 1000 1500 2000 2500 3000
Square Feet
HousePrice($1000s)
Example: Making Predictions
 When using a regression model for prediction,
only predict within the relevant range of data
Relevant range for
interpolation
Do not try to
extrapolate
beyond the range
of observed X’s

Measures of Variation
 Total variation is made up of two parts:
SSESSRSST 
Total Sum of
Squares
Regression Sum
of Squares
Error Sum of
Squares
  2
i )YY(SST   2
ii )YˆY(SSE  2
i )YYˆ(SSR
where:
= Mean value of the dependent variable
Yi = Observed value of the dependent variable
= Predicted value of Y for the given Xi valueiYˆ
Y

 SST = total sum of squares (Total Variation)
 Measures the variation of the Yi values around their
mean Y
 SSR = regression sum of squares (Explained Variation)
 Variation attributable to the relationship between X
and Y
 SSE = error sum of squares (Unexplained Variation)
 Variation in Y attributable to factors other than X
(continued)

(continued)
Xi
Y
X
Yi
SST = (Yi - Y)2
SSE = (Yi - Yi )2

SSR = (Yi - Y)2

_
_
_
Y

Y
Y
_
Y


 The coefficient of determination is the portion
of the total variation in the dependent variable
that is explained by variation in the
independent variable
 The coefficient of determination is also called
r-squared and is denoted as r2
Coefficient of Determination, r2
1r0 2
note:
squaresofsum
squaresofregression2
total
sum
SST
SSR
r 

r2 = 1
Examples of Approximate
r2 Values
Y
X
Y
X
r2 = 1
r2 = 1
Perfect linear relationship
between X and Y:
100% of the variation in Y is
explained by variation in X

r2 Values
Y
X
Y
X
0 < r2 < 1
Weaker linear relationships
between X and Y:
Some but not all of the
variation in Y is explained
by variation in X

r2 Values
r2 = 0
No linear relationship
between X and Y:
The value of Y does not
depend on X. (None of the
variation in Y is explained
by variation in X)
Y
X
r2 = 0

Coefficient of Determination, r2 in Excel
Multiple R 0.76211
R Square 0.58082
Observations 10
ANOVA
Regression 1 18934.9348 18934.9348 11.0848 0.01039
Residual 8 13665.5652 1708.1957
Total 9 32600.5000
Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386
Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580
58.08% of the variation in
house prices is explained by
variation in square feet
0.58082
32600.5000
18934.9348
SST
SSR
r2


Coefficient of Determination, r2 in Minitab
Constant 98.25 58.03 1.69 0.129
Square Feet 0.10977 0.03297 3.33 0.010
S = 41.3303 R-Sq = 58.1% R-Sq(adj) = 52.8%
Source DF SS MS F P
Regression 1 18935 18935 11.08 0.010
Total 9 32600
0.58082
32600.5000
18934.9348
SST
SSR
r2

58.08% of the variation in
house prices is explained
by variation in square feet

Standard Error of Estimate
 The standard deviation of the variation of
observations around the regression line is
estimated by
2
)ˆ(
2
1
2







n
YY
n
SSE
S
n
i
ii
YX
Where
SSE = error sum of squares
n = sample size

Standard Error of Estimate in Excel
Multiple R 0.76211
R Square 0.58082
Observations 10
ANOVA
Regression 1 18934.9348 18934.9348 11.0848 0.01039
Residual 8 13665.5652 1708.1957
Total 9 32600.5000
Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386
Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580
41.33032SYX 

Standard Error of Estimate in Minitab
Constant 98.25 58.03 1.69 0.129
Square Feet 0.10977 0.03297 3.33 0.010
S = 41.3303 R-Sq = 58.1% R-Sq(adj) = 52.8%
Source DF SS MS F P
Regression 1 18935 18935 11.08 0.010
Total 9 32600
41.33032SYX 

Comparing Standard Errors
YY
X X
YX
Ssmall
YX
Slarge
SYX is a measure of the variation of observed
Y values from the regression line
The magnitude of SYX should always be judged relative to the
size of the Y values in the sample data
i.e., SYX = $41.33K is moderately small relative to house prices in
the $200K - $400K range

Assumptions of Regression
L.I.N.E
 Linearity
 The relationship between X and Y is linear
 Independence of Errors
 Error values are statistically independent
 Normality of Error
 Error values are normally distributed for any given
value of X
 Equal Variance (also called homoscedasticity)
 The probability distribution of the errors has constant
variance

Residual Analysis
 The residual for observation i, ei, is the difference
between its observed and predicted value
 Check the assumptions of regression by examining the
residuals
 Examine for linearity assumption
 Evaluate independence assumption
 Evaluate normal distribution assumption
 Examine for constant variance for all levels of X
(homoscedasticity)
 Graphical Analysis of Residuals
 Can plot residuals vs. X
iii YˆYe 

Residual Analysis for Linearity
Not Linear Linear

x
residuals
x
Y
x
Y
x
residuals

Residual Analysis for
Independence
Not Independent
Independent
X
X
residuals
residuals
X
residuals


Checking for Normality
 Examine the Stem-and-Leaf Display of the
Residuals
 Examine the Boxplot of the Residuals
 Examine the Histogram of the Residuals
 Construct a Normal Probability Plot of the
Residuals

Residual Analysis for Normality
Percent
Residual
When using a normal probability plot, normal
errors will approximately display in a straight line
-3 -2 -1 0 1 2 3
0
100

Residual Analysis for
Equal Variance
Non-constant variance  Constant variance
x x
Y
x x
Y
residuals
residuals

House Price Model Residual Plot
-60
-40
-20
0
20
40
60
80
0 1000 2000 3000
Square Feet
Residuals
Example: Excel Residual Output
RESIDUAL OUTPUT
Predicted
House Price Residuals
1 251.92316 -6.923162
2 273.87671 38.12329
3 284.85348 -5.853484
4 304.06284 3.937162
5 218.99284 -19.99284
6 268.38832 -49.38832
7 356.20251 48.79749
8 367.17929 -43.17929
9 254.6674 64.33264
10 284.85348 -29.85348
Does not appear to violate
any regression assumptions

 Used when data are collected over time to
detect if autocorrelation is present
 Autocorrelation exists if residuals in one
time period are related to residuals in
another period
Measuring Autocorrelation:
The Durbin-Watson Statistic

Autocorrelation
 Autocorrelation is correlation of the errors
(residuals) over time
 Violates the regression assumption that
residuals are random and independent
Time (t) Residual Plot
-15
-10
-5
0
5
10
15
0 2 4 6 8
Time (t)
Residuals
 Here, residuals show a
cyclic pattern, not
random. Cyclical
patterns are a sign of
positive autocorrelation

The Durbin-Watson Statistic





 n
1i
2
i
n
2i
2
1ii
e
)ee(
D
 The possible range is 0 ≤ D ≤ 4
 D should be close to 2 if H0 is true
 D less than 2 may signal positive
autocorrelation, D greater than 2 may
signal negative autocorrelation
 The Durbin-Watson statistic is used to test for
autocorrelation
H0: residuals are not correlated
H1: positive autocorrelation is present

Testing for Positive
Autocorrelation
 Calculate the Durbin-Watson test statistic = D
(The Durbin-Watson Statistic can be found using Excel or Minitab)
Decision rule: reject H0 if D < dL
H0: positive autocorrelation does not exist
H1: positive autocorrelation is present
0 dU 2dL
Reject H0 Do not reject H0
 Find the values dL and dU from the Durbin-Watson table
(for sample size n and number of independent variables k)
Inconclusive

 Suppose we have the following time series
data:
 Is there autocorrelation?
y = 30.65 + 4.7038x
R
2
= 0.8976
0
20
40
60
80
100
120
140
160
0 5 10 15 20 25 30
Time
Sales
Autocorrelation
(continued)

 Example with n = 25:
Durbin-Watson Calculations
Sum of Squared
Difference of Residuals 3296.18
Sum of Squared
Residuals 3279.98
Durbin-Watson
Statistic 1.00494
y = 30.65 + 4.7038x
R
2
= 0.8976
0
20
40
60
80
100
120
140
160
0 5 10 15 20 25 30
Time
Sales
Autocorrelation
(continued)
Excel/PHStat output:
1.00494
3279.98
3296.18
e
)e(e
D n
1i
2
i
n
2i
2
1ii









 Here, n = 25 and there is k = 1 one independent variable
 Using the Durbin-Watson table, dL = 1.29 and dU = 1.45
 D = 1.00494 < dL = 1.29, so reject H0 and conclude that
significant positive autocorrelation exists
Autocorrelation
(continued)
Decision: reject H0 since
D = 1.00494 < dL
0 dU=1.45 2dL=1.29
Reject H0 Do not reject H0Inconclusive

Inferences About the Slope
 The standard error of the regression slope
coefficient (b1) is estimated by
 

2
i
YXYX
b
)X(X
S
SSX
S
S 1
where:
= Estimate of the standard error of the slope
= Standard error of the estimate
1bS
2n
SSE
SYX



Inferences About the Slope:
t Test
 t test for a population slope
 Is there a linear relationship between X and Y?
 Null and alternative hypotheses
 H0: β1 = 0 (no linear relationship)
 H1: β1 ≠ 0 (linear relationship does exist)
 Test statistic
1
b
11
STAT
S
βb
t


2nd.f. 
where:
b1 = regression slope
coefficient
β1 = hypothesized slope
Sb1 = standard
error of the slope

t Test Example
House Price
in $1000s
(y)
Square Feet
(x)
245 1400
312 1600
279 1700
308 1875
199 1100
219 1550
405 2350
324 2450
319 1425
255 1700
(sq.ft.)0.109898.25pricehouse 
Estimated Regression Equation:
The slope of this model is 0.1098
Is there a relationship between the
square footage of the house and its
sales price?

t Test Example
H0: β1 = 0
H1: β1 ≠ 0From Excel output:
Coefficients Standard Error t Stat P-value
Intercept 98.24833 58.03348 1.69296 0.12892
Square Feet 0.10977 0.03297 3.32938 0.01039
1bSb1
329383
032970
0109770
S
βb
t
1
b
11
STAT
.
.
.





Constant 98.25 58.03 1.69 0.129
Square Feet 0.10977 0.03297 3.33 0.010
From Minitab output:
b1 1bS

t Test Example
Test Statistic: tSTAT = 3.329
There is sufficient evidence
that square footage affects
house price
Decision: Reject H0
Reject H0Reject H0
a/2=.025
-tα/2
Do not reject H0
0
tα/2
a/2=.025
-2.3060 2.3060 3.329
d.f. = 10- 2 = 8
H0: β1 = 0
H1: β1 ≠ 0

t Test ExampleH0: β1 = 0
H1: β1 ≠ 0
From Excel output:
Coefficients Standard Error t Stat P-value
Intercept 98.24833 58.03348 1.69296 0.12892
Square Feet 0.10977 0.03297 3.32938 0.01039
p-value
There is sufficient evidence that
square footage affects house price.
Decision: Reject H0, since p-value < α
Constant 98.25 58.03 1.69 0.129
Square Feet 0.10977 0.03297 3.33 0.010
From Minitab output:

F Test for Significance
 F Test statistic:
where
MSE
MSR
FSTAT 
1kn
SSE
MSE
k
SSR
MSR



where FSTAT follows an F distribution with k numerator and (n – k - 1)
denominator degrees of freedom
(k = the number of independent variables in the regression model)

F-Test for Significance
Excel Output
Multiple R 0.76211
R Square 0.58082
Observations 10
ANOVA
Regression 1 18934.9348 18934.9348 11.0848 0.01039
Residual 8 13665.5652 1708.1957
Total 9 32600.5000
11.0848
1708.1957
18934.9348
MSE
MSR
FSTAT 
With 1 and 8 degrees
of freedom
p-value for
the F-Test

F-Test for Significance
Minitab Output
Source DF SS MS F P
Regression 1 18935 18935 11.08 0.010
Total 9 32600
11.0848
1708.1957
18934.9348
MSE
MSR
FSTAT With 1 and 8 degrees
of freedom
p-value for
the F-Test

H0: β1 = 0
H1: β1 ≠ 0
a = .05
df1= 1 df2 = 8
Test Statistic:
Decision:
Conclusion:
Reject H0 at a = 0.05
There is sufficient evidence that
house size affects selling price0
a = .05
F.05 = 5.32
Reject H0Do not
reject H0
11.08FSTAT 
MSE
MSR
Critical
Value:
Fa = 5.32
F Test for Significance
(continued)
F

Confidence Interval Estimate
for the Slope
Confidence Interval Estimate of the Slope:
Excel Printout for House Prices:
At 95% level of confidence, the confidence interval for
the slope is (0.0337, 0.1858)
1
b2/1 Sb αt
Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386
Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580
d.f. = n - 2

Since the units of the house price variable is
$1000s, we are 95% confident that the average
impact on sales price is between $33.74 and
$185.80 per square foot of house size
Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386
Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580
This 95% confidence interval does not include 0.
Conclusion: There is a significant relationship between
house price and square feet at the .05 level of significance
Confidence Interval Estimate
for the Slope
(continued)

t Test for a Correlation Coefficient
 Hypotheses
H0: ρ = 0 (no correlation between X and Y)
H1: ρ ≠ 0 (correlation exists)
 Test statistic
(with n – 2 degrees of freedom)
2n
r1
ρ-r
t
2
STAT



0bifrr
0bifrr
where
1
2
1
2



t-test For A Correlation Coefficient
Is there evidence of a linear relationship
between square feet and house price at the
.05 level of significance?
H0: ρ = 0 (No correlation)
H1: ρ ≠ 0 (correlation exists)
a =.05 , df = 10 - 2 = 8
3.329
210
.7621
0.762
2n
r1
ρr
t
22
STAT 








(continued)

t-test For A Correlation Coefficient
Conclusion:
There is
evidence of a
linear association
at the 5% level of
significance
Decision:
Reject H0
Reject H0Reject H0
a/2=.025
-tα/2
Do not reject H0
0
tα/2
a/2=.025
-2.3060 2.3060
3.329
d.f. = 10-2 = 8
3.329
210
.7621
0.762
2n
r1
ρr
t
22
STAT 








(continued)

Estimating Mean Values and
Predicting Individual Values
Y
XXi
Y = b0+b1Xi

Confidence
Interval for
the mean of
Y, given Xi
Prediction Interval
for an individual Y,
given Xi
Goal: Form intervals around Y to express
uncertainty about the value of Y for a given Xi
Y


Confidence Interval for
the Average Y, Given X
Confidence interval estimate for the
mean value of Y given a particular Xi
Size of interval varies according
to distance away from mean, X
ihtY YX2/
XX|Y
Sˆ
:μforintervalConfidence
i
α

 



 2
i
2
i
2
i
i
)X(X
)X(X
n
1
SSX
)X(X
n
1
h

Prediction Interval for
an Individual Y, Given X
Confidence interval estimate for an
Individual value of Y given a particular Xi
This extra term adds to the interval width to reflect
the added uncertainty for an individual case
ihtY 

1Sˆ
:YforintervalConfidence
YX2/
XX i
α

Estimation of Mean Values:
Example
Find the 95% confidence interval for the mean price
of 2,000 square-foot houses
Predicted Price Yi = 317.85 ($1,000s)

Confidence Interval Estimate for μY|X=X
37.12317.85
)X(X
)X(X
n
1
StYˆ
2
i
2
i
YX0.025 




The confidence interval endpoints are 280.66 and 354.90,
or from $280,660 to $354,900
i

Estimation of Individual Values:
Example
Find the 95% prediction interval for an individual
house with 2,000 square feet
Predicted Price Yi = 317.85 ($1,000s)

Prediction Interval Estimate for YX=X
102.28317.85
)X(X
)X(X
n
1
1StYˆ
2
i
2
i
YX0.025 




The prediction interval endpoints are 215.50 and 420.07,
or from $215,500 to $420,070
i

Finding Confidence and
Prediction Intervals in Excel
 From Excel, use
PHStat | regression | simple linear regression …
 Check the
“confidence and prediction interval for X=”
box and enter the X-value and confidence level
desired

Input values
Prediction Intervals in Excel
(continued)
Confidence Interval Estimate for μY|X=Xi
Prediction Interval Estimate for YX=Xi
Y


Prediction Intervals in Minitab
Predicted Values for New Observations
New
Obs Fit SE Fit 95% CI 95% PI
1 317.8 16.1 (280.7, 354.9) (215.5, 420.1)
Values of Predictors for New Observations
New Square
Obs Feet
1 2000
Y

Input values
Confidence Interval Estimate for μY|X=Xi
Prediction Interval Estimate for YX=Xi

Pitfalls of Regression Analysis
 Lacking an awareness of the assumptions
underlying least-squares regression
 Not knowing how to evaluate the assumptions
 Not knowing the alternatives to least-squares
regression if a particular assumption is violated
 Using a regression model without knowledge of
the subject matter
 Extrapolating outside the relevant range

Strategies for Avoiding
the Pitfalls of Regression
 Start with a scatter plot of X vs. Y to observe
possible relationship
 Perform residual analysis to check the
assumptions
 Plot the residuals vs. X to check for violations of
assumptions such as homoscedasticity
 Use a histogram, stem-and-leaf display, boxplot,
or normal probability plot of the residuals to
uncover possible non-normality

Strategies for Avoiding
the Pitfalls of Regression
 If there is violation of any assumption, use
alternative methods or models
 If there is no evidence of assumption violation,
then test for the significance of the regression
coefficients and construct confidence intervals
and prediction intervals
 Avoid making predictions or forecasts outside
the relevant range
(continued)

Chapter Summary
 Introduced types of regression models
 Reviewed assumptions of regression and
correlation
 Discussed determining the simple linear
regression equation
 Described measures of variation
 Discussed residual analysis
 Addressed measuring autocorrelation

Chapter Summary
 Described inference about the slope
 Discussed correlation -- measuring the strength
of the association
 Addressed estimation of mean values and
prediction of individual values
 Discussed possible pitfalls in regression and
recommended strategies to avoid them
(continued)

Bbs11 ppt ch13

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Bbs11 ppt ch13