CHAPTER 9 Summarizing and Graphing Your Data 123

Structuring Numerical Summaries

into Descriptive Tables

Now you know how to calculate the basic summary statistics that convey the gen-

eral idea of how a set of numerical values is distributed. So which summary sta-

tistics do you report? Generally, you select a few of the most useful summary

statistics in summarizing your particular data set, and arrange them in a concise

way. Many biostatisticians choose to report N, mean, SD, median, minimum, and

maximum, and arrange them something like this:

mean

SD

N

median minimum

maximum

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 sta-

tistics. Remember not to display decimals beyond what were collected in the orig-

inal data. Using this arrangement, the numbers would be reported this way:

98 3

14 4

.

.

7

91

84

116

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 partici-

pants 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)