Bar Charts
Prerequisites
Graphing Qualitative
Variables
Learning Objectives
- Create and interpret bar charts
- 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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