114 PART 3 Getting Down and Dirty with Data

Summarizing Numerical Data

Summarizing a numerical variable isn’t as simple as summarizing a categorical

variable. The summary statistics for a numerical variable should convey how the

individual values of that variable are distributed across your sample in a concise

and meaningful way. These summary statistics should give you some idea of the

shape of the true distribution of that variable in the population from which you

draw your sample (read Chapter 3 and Chapter 6 to refresh your memory about

sampling). That true population distribution can have almost any shape, includ-

ing the typical shapes shown in Figure 9-2: normal, skewed, pointy-topped, and

bimodal (two-peaked).

FIGURE 9-2:

Four different

shapes of

distributions:

normal (a),

skewed (b),

pointy-topped (c),

and bimodal

(two-peaked) (d).

© John Wiley & Sons, Inc.

FIGURE 9-1:

A frequency

bar chart (a)

and pie chart (b).

© John Wiley & Sons, Inc.