Chapter 2: Graphs, Charts, and Tables--Describing Your Data

46 CHAPTER 2 • GRAPHS, CHARTS, AND TABLES—DESCRIBING YOUR DATA
FIGURE 2.6
Minitab Output of
Credit Card Balance
Histogram
Minitab Instructions:
1. Open file: Capital.MTW.
2. Choose Graph >
Histogram.
3. Click Simple.
4. Click OK.
5. In Graph variables,
enter data column.
6. Click OK.
Issues with Excel If you use Excel to construct a histogram as indicated in the instructions
in Figure 2.5, the initial graph will come up with gaps between the bars. Because histograms
illustrate the distribution of data across the range of all possible values for the
quantitative variable, histograms do not have gaps. Therefore, to get the proper histogram
format, you need to close these gaps by setting the gap width to zero, as indicated in the
Excel instructions shown in Figure 2.5. Minitab provides no gaps with its default output,
as shown in Figure 2.6.
SUMMARY Constructing Frequency Histograms
To construct a frequency histogram, perform the following
steps:
1. Follow the steps for constructing a frequency distribution
(see Examples 2-1 or 2-3).
2. Use the horizontal axis to represent the variable of
interest. Use the vertical axis to represent the frequency
in each class.
3. Draw vertical bars for each class or data value so that
the heights of the bars correspond to the frequencies.
Make sure there are no gaps between the bars. (Note, if
the classes do not have equal widths, the bar height
should be adjusted to make the area of the bar proportional
to the frequency.)
4. Label the histogram appropriately.
EXAMPLE 2-4 Frequency Histograms
TRY PROBLEM 2.10
Emergency Response Times The director of emergency responses in Montreal,
Canada, is interested in analyzing the time needed for response teams to reach their destinations
in emergency situations after leaving their stations. She has acquired the response
times for 1,220 calls last month. To develop the frequency histogram, perform the following
steps: