- 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. For example, the bar chart shown in Figure
1 shows how many purchasers of iMac computers were previous Macintosh
users, previous Windows users, and new computer purchasers.
Figure 1. iMac buyers as a function of previous computer
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 2 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 2. Percent increase in three stock indexes from May 24th
2000 to May 24th 2001.
Bar charts are particularly effective for showing
change over time. Figure 3, for example, shows the percent increase
in the consumer price index (CPI) over four three-month periods.
The fluctuation in inflation is apparent in the graph.
Figure 3. Percent change in the CPI over time. Each bar
represents percent increase for the three months ending at the date indicated.
Bar charts are often used to compare the means of different
experimental conditions. Figure 4 shows the mean time it took one of us (DL) to
move the mouse to either a small target or a large target. On the average more
time was required for small targets than for large ones.
Figure 4. Bar chart showing the means for the two 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. For example, a box plot of the mouse-movement
data is shown in Figure 5. You can see that Figure 5 reveals more
about the distribution of movement-times than does Figure 4.
Figure 5. Box plots of times to move the mouse to the small
and large targets.
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
to the use of bar charts to display quantitative variables.