Linear Regression
Analysis
Scatter Plots and Correlation
• A scatter plot (or scatter diagram) is used to
show the relationship between two variables
• Correlation analysis is used to measure
strength of the association (linear
relationship) between two variables
– Only concerned with strength of the
relationship
– No causal effect is implied
Scatter Plot Examples
Linear relationships
y
Nonlinear relationships
y
x
y
x
y
x
x
Scatter Plot Examples
(continued)
Strong relationships
y
Weak relationships
y
x
y
x
y
x
x
Scatter Plot Examples
(continued)
No relationship
y
x
y
x
Correlation Coefficient
• The sample correlation coefficient r is
used to measure the strength of the
linear relationship in the sample
observations
Features of r
• Range between -1 and 1
• The closer to -1, the stronger the
negative linear relationship
• The closer to 1, the stronger the positive
linear relationship
• The closer to 0, the weaker the linear
relationship
Examples of Approximate
r Values
y
y
y
x
r=-1
r = - 0.6
y
x
x
r=0
y
r = 0.3
x
r = +1
x
Calculating the
Correlation Coefficient
Sample correlation coefficient:
r
 ( x  x)( y  y)
[ ( x  x ) ][  ( y  y ) ]
2
2
or the algebraic equivalent:
r
n xy   x  y
[n( x 2 )  ( x )2 ][n(  y 2 )  (  y )2 ]
where:
r = Sample correlation coefficient
n = Sample size
x = Value of the explanatory variable
y = Value of the response variable
Calculation Example
Tree
Height
Trunk
Diameter
y
x
xy
y2
x2
35
8
280
1225
64
49
9
441
2401
81
27
7
189
729
49
33
6
198
1089
36
60
13
780
3600
169
21
7
147
441
49
45
11
495
2025
121
51
12
612
2601
144
=321
=73
=3142 =14111
=713
Calculation Example
Tree Height,
y
r
70
n xy   x  y
[n(  x 2 )  (  x) 2 ][n(  y 2 )  (  y) 2 ]
60

50
40
(continued)
8(3142)  (73)(321)
[8(713)  (73) 2 ][8(14111)  (321)2 ]
 0.886
30
20
10
0
0
2
4
6
8
10
Trunk Diameter, x
12
14
r = 0.886 → relatively strong positive
linear association between x and y
Introduction to Regression Analysis
• Regression analysis is used to:
– Predict the value of a response variable based
on the value of at least one explanatory variable
– Explain the impact of changes in an explanatory
variable on the response variable
Response variable: the variable we wish to
explain
Explanatory variable: the variable used to
explain the response variable
Simple Linear Regression
Model
• Only one explanatory variable, x
• Relationship between x and y is
described by a linear function
• Changes in y are assumed to be
caused by changes in x
Types of Regression Models
Positive Linear Relationship
Negative Linear Relationship
Relationship NOT Linear
No Relationship
Regression
• Fitting the data: finding the equation for the straight
line that does the best job of reproducing the data.
Average Income versus % with a College Degree (by State)
Average Income Level ($ per year)
40,000
35,000
30,000
25,000
20,000
15,000
10
15
20
25
Percentage of Population with
College Degree or Higher
30
35
Regression
• Residual: Difference between measured and
calculated Y-values – let’s call it ‘e’
Average Income versus % with a College Degree (by State)
Average Income Level ($ per year)
26,000
25,500
25,000
24,500
24,000
23,500
23,000
22,500
22,000
15
15.5
16
16.5
17
17.5
18
Percentage of Population with
College Degree or Higher
18.5
19
19.5
20
Regression
• The sum of the residuals always equals 0.
Average Income versus % with a College Degree (by State)
Average Income Level ($ per year)
26,000
25,500
25,000
24,500
24,000
23,500
23,000
22,500
22,000
15
15.5
16
16.5
17
17.5
18
Percentage of Population with
College Degree or Higher
18.5
19
19.5
20
Simple linear regression uses
the least regression method.
A least squares regression line
is the line which produces the
smallest value of the sum of the
squares of the residuals
The Least Squares Equation
• The formula for b is:
x y

xy
a

y

b
x
algebraic
n
equivalent:
b
2
( x)
2
x 
n




