Different types of Variables
What is the difference between nominal, ordinal
and scale?
In SPSS, we can specify the level of measurement as scale
(numeric data on an interval or ratio scale), ordinal, or nominal. Nominal and
ordinal data can be either string alphanumeric) or numeric. But what is the difference?
Nominal or categorical,
ordinal and interval
In terms of variables, they are described as
categorical (or sometimes nominal), or ordinal, or interval. Let’s
see the definition and look why they are important.
Categorical or
Nominal
A categorical variable (sometimes called a nominal
variable) is one that has two or more categories, but there is no intrinsic
ordering to the categories. Examples are given below-
a.
Gender is a categorical variable having two
categories (male and female) and there is no intrinsic ordering to the
categories.
b.
Hair color is also a categorical variable having a
number of categories (blonde, brown, brunette, red, etc.) and again, there is
no agreed way to order these from highest to lowest.
c.
Examples of nominal variables include region, zip
code, or religious affiliation
A purely
categorical variable is one that simply allows you to assign categories but you
cannot clearly order the variables.
Ordinal
An ordinal variable is similar to a categorical
variable. The difference between the two is that there is a clear
ordering of the variables. Examples are given below:-
a.
Economic status, with three categories (low, medium
and high). We can classify people into these three categories, also can
order the categories as low, medium and high.
b.
Educational experience (with values such as
elementary school graduate, high school graduate, some college and college
graduate). These also can be ordered as elementary school, high school, some college,
and college graduate.
c.
Examples of ordinal variables also include attitude
scores representing degree of satisfaction or confidence and preference rating
scores.
Interval or
scale
An interval variable is similar to an ordinal
variable, except that the intervals between the values of the interval variable
are equally spaced. Examples are given below:-
a.
Annual income is a variable that is measured in
dollars, and we have three people who make $10,000, $15,000 and $20,000. The
second person makes $5,000 more than the first person and $5,000 less than the
third person, and the size of these intervals is the same. If there
were two other people who make $90,000 and $95,000, the size of that interval
between these two people is also the same ($5,000).
b. Examples of scale variables also include age in
years.
Why does it matter
whether a variable is categorical, ordinal or interval?
Statistical
computations and analyses assume that the variables have a specific levels of
measurement. For example, it would not make sense to compute an average
hair color. An average of a categorical variable does not make much sense
because there is no intrinsic ordering of the levels of the categories.
Moreover, if you tried to compute the average of educational experience as
defined in the ordinal section above, you would also obtain a nonsensical
result. Because the spacing between the four levels of educational
experience is very uneven, the meaning of this average would be very
questionable. In short, an average requires a variable to be
interval. Sometimes you have variables that are "in between"
ordinal and interval, for example, a five-point Likert scale with values
"strongly agree", "agree", "neutral",
"disagree" and "strongly disagree". If we cannot be
sure that the intervals between each of these five values are the same, then we
would not be able to say that this is an interval variable, but we would say
that it is an ordinal variable. However, in order to be able to use
statistics that assume the variable is interval, we will assume that the
intervals are equally spaced.