Bar Charts


David M. Lane


Graphing Qualitative Variables

Learning Objectives
  1. Create and interpret bar charts
  2. Judge whether a bar chart or another graph such as a box plot would be more appropriate

In the section on qualitative variables, we saw how bar charts could be used to illustrate the frequencies of different categories. In this section we show how bar charts can be used to present other kinds of quantitative information, not just frequency counts. The bar chart in Figure 1 shows the percent increases in the Dow Jones, Standard and Poor 500 (S & P), and Nasdaq stock indexes from May 24th 2000 to May 24th 2001. Notice that both the S & P and the Nasdaq had “negative increases” which means that they decreased in value. In this bar chart, the Y-axis is not frequency but rather the signed quantity percentage increase.

Figure 1. Percent increase in three stock indexes from May 24th 2000 to May 24th 2001.

Bar charts are particularly effective for showing change over time. Bar charts are often used to compare the means of different experimental conditions. Although bar charts can display means, we do not recommend them for this purpose. Box plots should be used instead since they provide more information than bar charts without taking up more space. The section on qualitative variables earlier in this chapter discussed the use of bar charts for comparing distributions. Some common graphical mistakes were also noted. The earlier discussion applies equally well here, to the use of bar charts to display quantitative variables.

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