Читать книгу Applied Biostatistics for the Health Sciences - Richard J. Rossi - Страница 49

Qualitative variables take on nonnumeric values and are usually used to represent a distinct quality of a population unit. When the possible values of a qualitative variable have no intrinsic ordering, the variable is called a nominal variable; when there is a natural ordering of the possible values of the variable, then the variable is called an ordinal variable. An example of a nominal variable is Blood Type where the standard values for blood type are A, B, AB, and O. Clearly, there is no intrinsic ordering of these blood types, and hence, Blood Type is a nominal variable. An example of an ordinal variable is the variable Pain where a subject is asked to describe their pain verbally as

 No pain,

 Mild pain,

 Discomforting pain,

 Distressing pain,

 Intense pain,

 Excruciating pain.

In this case, since the verbal descriptions describe increasing levels of pain, there is a clear ordering of the possible values of the variable Pain levels, and therefore, Pain is an ordinal qualitative variable.

Example 2.2

In the Framingham Heart Study of coronary heart disease, the following two nominal qualitative variables were recorded:

and

Example 2.3

An example of an ordinal variable is the variable Baldness when measured on the Norwood–Hamilton scale for male-pattern baldness. The variable Baldness is measured according to the seven categories listed below:

1 I Full head of hair without any hair loss.

2 II Minor recession at the front of the hairline.

3 III Further loss at the front of the hairline, which is considered “cosmetically significant.”

4 IV Progressively more loss along the front hairline and at the crown.

5 V Hair loss extends toward the vertex.

6 VI Frontal and vertex balding areas merge into one and increase in size.

7 VII All hair is lost along the front hairline and crown.

Clearly, the values of the variable Baldness indicate an increasing degree of hair loss, and thus, Baldness as measured on the Norwood–Hamilton scale is an ordinal variable. This variable is also measured on the Offspring Cohort in the Framingham Heart Study.