124 PART 3 Getting Down and Dirty with Data

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 treat-

ment, 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 commu-

nicating 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 vari-

able, as shown in Figure 9-5.

FIGURE 9-5:

Population

distribution of

systolic blood

pressure (SBP)

measurements in

mmHg (a) and

distribution of a

sample from that

population (b).

© John Wiley & Sons, Inc.