60 CHAPTER 2 Descriptive Statistics Graphing Paired Data Sets When each entry in one data set corresponds to one entry in a second data set, the sets are called paired data sets. For instance, a data set contains the costs of an item and a second data set contains sales amounts for the item at each cost. Because each cost corresponds to a sales amount, the data sets are paired. One way to graph paired data sets is to use a scatter plot, where the ordered pairs are graphed as points in a coordinate plane. A scatter plot is used to show the relationship between two quantitative variables. Interpreting a Scatter Plot The British statistician Ronald Fisher (see page 35) introduced a famous data set called Fisher’s Iris data set. This data set describes various physical characteristics, such as petal length and petal width (in millimeters), for three species of iris. In the scatter plot shown, the petal lengths form the first data set and the petal widths form the second data set. As the petal length increases, what tends to happen to the petal width? (Source: Fisher, R. A., 1936) 10 20 30 40 50 60 70 5 10 15 20 25 Petal length (in millimeters) Petal width (in millimeters) Fisher’s Iris Data Set SOLUTION The horizontal axis represents the petal length, and the vertical axis represents the petal width. Each point in the scatter plot represents the petal length and petal width of one flower. Interpretation From the scatter plot, you can see that as the petal length increases, the petal width also tends to increase. TRY IT YOURSELF 6 The lengths of employment and the salaries of 10 employees are listed in the table below. Graph the data using a scatter plot. Describe any trends. Length of employment (in years) 5 4 8 4 2 Salary (in dollars) 36,000 36,500 45,000 31,350 28,000 Length of employment (in years) 10 7 6 9 3 Salary (in dollars) 48,000 46,650 44,225 50,100 32,000 Answer: Page A36 You will learn more about scatter plots and how to analyze them in Chapter 9. EXAMPLE 6
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