The Essentials of Biostatistics for Physicians, Nurses, and Clinicians

3.6 Bar and Pie Charts 41

there is no natural order exhibited. The bar chart can also be used when

there is no natural order, but it is easy for the viewer to think that, since

the bars go from left to right, the bar chart, like the histogram, is dis-

playing the categories in increasing order.

In Figure 3.3 , the Pugh data provides a measure of severity of liver

disease. The Pugh categories run from 1 to 7 in increasing level of

severity. Here is a bar chart for the Pugh data.

Note that the bar chart looks just like a relative frequency histo-

gram, but remember that the numbers represent categories and not

intervals of real numbers. So a 2 is not twice as severe as a 1, for

example. But as we move from left to right, the severity increases. So,

for the Pugh, data the bar chart is appropriate

Next, we shall look at the same data viewed as a pie chart (Fig. 3.4 ).

It is much easier to identify the differences in proportions visually

from the pie chart. The order is lost unless you recognize that order of

severity starts with 1 in the upper right quadrant and increases as you

move clockwise from there. For data like this, it may be useful to

present both types of graphs so that the viewer will recognize both

features clearly. But had there not been a natural ordering to the data,

only the pie chart should be used.

Figure 3.3. Relative frequency bar graph for Pugh categories of 24 pediatric patients
with liver disease.

30

25

20

15

10

5

0

1234567

Pugh Category

Relative Frequency (%)