Statistical Methods for Psychology

250 Chapter 9 Correlation and Regression

4.2

–2 –1 0

Theoretical quantiles

Loge symptoms

12

4.4

4.6

4.8

5.0

Sample quantiles

Table 9.1 Description of data on the relationship between stress and mental health

LnSymptoms

The decimal point is 1 digit(s) to the left of the |

40 6

41 11334

41 67799

42 2

42 5556899

43 0000244

43 66677888999

44 111222334

44 555577888899

45 0111223344

45 55667

46 00001112222224

46 567799

47 112

47 67

48 0034

48 8

49 11

49 89

Stress

The decimal point is 1 digit(s) to the right of the |

0 1123334

0 5567788899999

1 011222233333444

1 555555566667778889

2 0000011222223333444

2 56777899

3 0013334444

3 66778889

4 334

4 5555

5

558

20

10

0

–2 –1 0

Theoretical quantiles

Stress

12

30

40

50

60

Sample quantiles

9.3 The Covariance

The correlation coefficient we seek to compute on the data^4 in Table 9.2 is itself based on a

statistic called the covariance (covXYor sXY). The covariance is basically a number that

reflects the degree to which two variables vary together.

(^4) A copy of the complete data set is available on this book’s Web site in the file named Table 9.1.dat.
covariance
(covXYor sXY)