Introductory Biostatistics

Example 2.5 The distribution of family income for the United States in 1983
by race is shown in Table 2.5. The distribution for nonwhite families is repre-
sented in the histogram in Figure 2.5, where the vertical axis represents the
density (percent per $1000). It is obvious that the distribution is not symmetric;
it is very skewed to the right. In this histogram, we graph the densities on the
vertical axis. For example, for the second income interval (15,000–19,999), the
relative frequency is 31% and the width of the interval is $5000 (31% per
$5000), leading to the density

31

5000

 1000 ¼ 6 : 2

or 6.2% per $1000 (we arbitrarily multiply by 1000, or any power of 10, just to
obtain a larger number for easy graphing).

TABLE 2.5

Percent of Families

Income ($) White Nonwhite

0–14,999 13 34

15,000–19,999 24 31

20,000–24,999 26 19

25,000–34,999 28 13

35,000–59,999 9 3

60,000 and over 1 Negligible

Total 100 100

Figure 2.5 Income of U.S. nonwhite families, 1983.

66 DESCRIPTIVE METHODS FOR CONTINUOUS DATA