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Chapter 18: A. Basic Statistics and Applications 227
Continued
Table 18.8
Cholesterol levels and systolic blood pressures of 30 randomly selected
U.S. males.
Subject 1 2 3 4 5 6 7 8 9 10
Cholesterol (x) 195 180 220 160 200 220 200 183 139 155
Systolic BP (y) 130 128 138 122 140 148 142 127 116 123
Subject 11 12 13 14 15 16 17 18 19 20
Cholesterol (x) 153 164 171 143 159 167 162 165 178 145
Systolic BP (y) 119 130 128 120 121 124 118 121 124 115
Subject 21 22 23 24 25 26 27 28 29 30
Cholesterol (x) 245 198 156 175 171 167 142 187 158 142
Systolic BP (y) 145 126 122 124 117 122 112 131 122 120
150
140
r = .891
y
130
120
110
150 175
200 225 250
x
Part IV.A.4
Figure 18.14 Scatter plot for the data in Table 18.8.
Karl Pearson (1857–1936). The correlation coefficient between two numerical variables
in sample data is usually denoted by r. The Greek letter r (rho) denotes the corresponding
measure of association that is the correlation coefficient for a population of data.
The correlation coefficient is defined as
r =
( xi
x) ( yi
y)
( x x) ( y y)
i
2 2
i
=
( x
) i ( yi
)
xy
i i
n
( x
)
2
2 i
2
yi
xi
yi
n n
( )
2
. (18.16)
Continued