
CATEGORICAL RESPONSE DATA 3
An inter®al ®ariable is one that does have numerical distances between any
two values. For example, blood pressure level, functional life length of
television set, length of prison term, and annual income are interval vari-
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ables. An internal variable is sometimes called a ratio ®ariable if ratios of
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values are also valid.
The way that a variable is measured determines its classification. For
example, ‘‘education’’ is only nominal when measured as public school or
private school; it is ordinal when measured by highest degree attained, using
the categories none, high school, bachelor’s, master’s, and doctorate; it is
interval when measured by number of years of education, using the integers
0, 1, 2, . . . .
A variable’s measurement scale determines which statistical methods are
appropriate. In the measurement hierarchy, interval variables are highest,
ordinal variables are next, and nominal variables are lowest. Statistical
methods for variables of one type can also be used with variables at higher
levels but not at lower levels. For instance, statistical methods for nominal
variables can be used with ordinal variables by ignoring the ordering of
categories. Methods for ordinal variables cannot, however, be used with
nominal variables, since their categories have no meaningful ordering. It is
usually best to apply methods appropriate for the actual scale.
Since this book deals with categorical responses, we discuss the analysis of
nominal and ordinal variables. The methods also apply to interval variables
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having a small number of distinct values e.g., number of times married or
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for which the values are grouped into ordered categories e.g., education
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measured as - 10 years, 10᎐12 years, ) 12 years .
1.1.3 Continuous–Discrete Variable Distinction
Variables are classified as continuous or discrete, according to the number of
values they can take. Actual measurement of all variables occurs in a discrete
manner, due to precision limitations in measuring instruments. The continu-
ous᎐discrete classification, in practice, distinguishes between variables that
take lots of values and variables that take few values. For instance, statisti-
cians often treat discrete interval variables having a large number of values
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such as test scores as continuous, using them in methods for continuous
responses.
Ž.
This book deals with certain types of discretely measured responses: 1
Ž. Ž.
nominal variables, 2 ordinal variables, 3 discrete interval variables having
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relatively few values, and 4 continuous variables grouped into a small
number of categories.
1.1.4 Quantitative–Qualitative Variable Distinction
Nominal variables are qualitati®eᎏdistinct categories differ in quality, not in
quantity. Interval variables are quantitati®eᎏdistinct levels have differing
amounts of the characteristic of interest. The position of ordinal variables in