Page 73 - Statistics II for Dummies

Chapter 4: Getting in Line with Simple Linear Regression 57

Table 4-1 Average Textbook Weight and Student Weight
(Grades 1–12)
Grade Average Student Average Textbook
Weight (In Pounds) Weight (In Pounds)
1 48.50 8.00
2 54.50 9.44
3 61.25 10.08
4 69.00 11.81
5 74.50 12.28
6 85.00 13.61
7 89.00 15.13
8 99.00 15.47
9 112.00 17.36
10 123.00 18.07
11 134.00 20.79
12 142.00 16.06

In this section, you begin exploring whether or not a relationship exists
between these two quantitative variables. You start by displaying the pairs of
data using a two-dimensional scatterplot to look for a possible pattern, and
you quantify the strength and direction of that pattern using the correlation
coefficient.

Using scatterplots to explore relationships

In order to explore a possible relationship between two variables, such as
textbook weight and student weight, you first plot the data in a special graph
called a scatterplot. A scatterplot is a two-dimensional graph that displays
pairs of data, one pair per observation in the (x, y) format. Figure 4-1 shows a
scatterplot of the textbook-weight data from Table 4-1.

09_466469-ch04.indd 57 7/24/09 10:20:35 AM