Introductory Biostatistics

an apparent bimodal distribution; however, a closer examination shows that
among the nine patients with over 100% saturation, eight (or 89%) are over 50
years of age. On the other hand, only four of 22 (or 18%) patients with less
than 100% saturation are over 50 years of age. The two peaks in Figure 2.4
might correspond to the two age groups.

2. Another application concerns the symmetry of the distribution as de-
   picted by the table or its graphs. A symmetric distribution is one in
   which the distribution has the same shape on both sides of the peak
   location. If there are more extremely large values, the distribution is then
   skewed to the right, or positively skewed. Examples include family
   income, antibody level after vaccination, and drug dose to produce a
   predetermined level of response, among others. It is common that for
   positively skewed distributions, subsequent statistical analyses should be
   performed on the log scale: for example, to compute and/or to compare
   averages of log(dose).

TABLE 2.4

Percent Percent Percent
Age Saturation Age Saturation Age Saturation

23 40 55 137 48 78

31 86 31 88 27 80

58 111 20 88 32 47

25 86 23 65 62 74

63 106 43 79 36 58

43 66 27 87 29 88

67 123 63 56 27 73

48 90 59 110 65 118

29 112 53 106 42 67

26 52 66 110 60 57

64 88

Figure 2.4 Frequency polygon for percentage saturation of bile in men.

TABULAR AND GRAPHICAL METHODS 65