CHAPTER 9 Summarizing and Graphing Your Data 113

After looking at the frequencies in Table 9-2, you may be curious about the per-

centages, which would make these numbers more comparable. But a cross-tab

can get very cluttered if you try to include them, as there are different types: the

column percentage, the row percentage, and the total percentage. For example,

the 60 rural residents with commercial health insurance in Table 9-2 comprise

46.9 percent of all participants with commercial health insurance, because

60 divided by the total number with commercial health insurance, which is 128

(the column total), equals 46.9 percent.

Groups are often compared across columns, and if that is the intention, column

percentages should be displayed. But if you divide these same 60 rural residents

with commercial insurance by their row total of 169 rural residents, you find they

make up 30.6 percent of all rural residents, which is a row percentage. And if you

go on to divide these 60 participants by the total sample size of the study, which

is 422, you find that they make up 14.2 percent of all participants in the study.

Categorical data are typically displayed graphically as frequency bar charts and as

pie charts:»

» ^{Frequency bar charts: Displaying the spread of participants across}

the different categories of a variable is commonly done by a bar chart

(see Figure 9-1a). Generally, statistical programs are used to make bar charts.

To create a bar chart manually from a tally of participants in each category,

you draw a graph containing one vertical bar for each category, making the

height proportional to the number of participants in that category.»

» ^{Pie charts: Pie charts indicate the relative number of participants in each}

category by the angle of a circular wedge, which can also be considered more

deliciously as a piece of the pie. To create a pie chart manually, you multiply

the percentage of participants in each category by 360, which is the number

of degrees of arc in a full circle, and then divide by 100. By doing that, you are

essentially figuring out what proportion of the circle to devote to that pie

piece. Next, you draw a circle with a compass, and then split it up into wedges

using a protractor — remember from high school math? Trust us, it’s easier to

use statistical software.

Most scientific writers recommend the usage of bar charts over pie charts. They

express more information in a smaller space, and allow for more accurate visual

comparisons.