Consider the example used earlier in this chapter of seven measures of diastolic blood pressure (DBP)

from a sample of study participants (with the values of 84, 84, 89, 91, 110, 114, and 116 mmHg),

where you calculated all these summary statistics. Remember not to display decimals beyond what

were collected in the original data. Using this arrangement, the numbers would be reported this way:

The real utility of this kind of compact summary is that you can place it in each cell of a table to show

changes over time and between groups. For example, a sample of systolic blood pressure (SBP)

measurements taken from study participants before and after treatment with two different hypertension

drugs (Drug A and Drug B) can be summarized concisely, as shown in Table 9-3.

TABLE 9-3 Systolic Blood Pressure Treatment Results

Before Treatment

After Treatment

Change

Mean ± SD (N)

Median (min – max) Mean ± SD (N)

Median (min – max) Mean ± SD (N) Median (min – max)

Drug A 138.7 ± 10.3 (40) 139.5 (117 – 161)

121.1 ± 13.9 (40) 121.5 (85 – 154)

-17.6 ± 8.0 (40) –17.5 (–34 – 4)

Drug B 141.0 ± 10.8 (40) 143.5 (111 – 160)

141.0 ± 15.4 (40) 142.5 (100 – 166)

-0.1 ± 9.9 (40)

1.5 (–25 – 18)

Table 9-3 shows that Drug A tended to lower blood pressure by about 18 mmHg. For Drug A, mean

SBP changed from 139 to 121 mmHg from before to after treatment, whereas the Drug B group

produced no noticeable change in blood pressure because it stayed around 141 mmHg from

pretreatment to post-treatment. All that’s missing are some p values to indicate the significance of the

changes over time within each group and of the differences between the groups. We show you how to

calculate those in Chapter 11.

Graphing Numerical Data

Displaying information graphically is a central part of interpreting and communicating the results of

scientific research. You can easily spot subtle features in a graph of your data that you’d never notice

in a table of numbers. Entire books have been written about graphing numerical data, so we only give a

brief summary of some of the more important points here.

Showing the distribution with histograms

Histograms are bar charts that show what fraction of the participants have values falling

within specified intervals called classes. The main purpose of a histogram is to show you how

the values of a numerical value are distributed. This distribution is an approximation of the true

population frequency distribution for that variable, as shown in Figure 9-5.