38 Chapter 2 Graphical Summaries of Data Table 2.3 Relative Frequency Distribution for Computer Sales Type of Computer Frequency Relative Frequency Desktop 11 0.22 Laptop 23 0.46 Notebook 9 0.18 Tablet 7 0.14 Check Your Understanding 1. The following table lists the types of aircraft for the landings that occurred during a day at a small airport. (“Single” refers to single-engine and “Twin” refers to twin-engine.) Types of Aircraft Landing at an Airport Twin Single Helicopter Turboprop Twin Single Turboprop Jet Jet Turboprop Turboprop Single Jet Single Single Twin Twin Turboprop Helicopter Single Single Single Twin Single Jet Twin Twin Single Twin Twin a. Construct a frequency distribution for these data. b. Construct a relative frequency distribution for these data. Answers are on page 47. Objective 2 Construct bar graphs Bar Graphs A bar graph is a graphical representation of a frequency distribution. A bar graph consists of rectangles of equal width, with one rectangle for each category. The heights of the rectangles represent the frequencies or relative frequencies of the categories. Example 2.3 shows how to construct a bar graph for the computer sales data in Table 2.3. EXAMPLE 2.3 Constructing bar graphs Construct a frequency bar graph and a relative frequency bar graph for the computer sales data in Table 2.3. Solution Step 1: Construct a horizontal axis. Place the category names along this axis, evenly spaced. Step 2: Construct a vertical axis to represent the frequency or the relative frequency. Step 3: Construct a bar for each category, with the heights of the bars equal to the frequencies or relative frequencies of their categories. The bars should not touch and should all be of the same width. Figure 2.1(a) presents a frequency bar graph, and Figure 2.1(b) presents a relative frequency bar graph. The graphs are identical except for the scale on the vertical axis. (a) 25 20 15 10 5 0 Desktop Laptop Notebook Tablet Frequency 0.5 0.4 0.3 0.2 0.1 0 Desktop Laptop Notebook Tablet (b) Relative Frequency Figure 2.1 (a) Frequency bar graph. (b) Relative frequency bar graph.
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