Nominal measurement

In deciding about the level of measurement represented by some set of data, first ask: Are the data in the form of words or are they represented by numbers? Data recorded in words, such as descriptions of actions, are considered as qualitative: Data in the form of numbers represent quantitative measurement. Qualitative data may be either in nominal or ordinal form. To separate the two, one needs to ask a second question: Can the data be ordered on some criteria?   If the answer is "no," the data were measured at the nominal level. Nominal measurement is shown at the left in Figure 6.3. Nominal comes from the Latin word "name;" with this level of measurement, the attributes of an indicator are known only in terms of the names of the categories they represent. Examples are gender, marital status, and nationality. For this kind of indicator, one can say that each of the characteristics is different and that is all.

Ordinal measurement

If the data are qualitative but the categories making up the variable can be ordered according to some criteria, the data are measured at the ordinal level. For example, suppose you asked respondents to indicate the level of schooling they had completed, using categories such as "none," "primary," "intermediate," and "secondary or higher." These levels represent ordinal measurement because respondents can be placed in some kind of rank order, from low to high.

Responses for many questionnaire items are obtained at the ordinal level. Respondents, for instance, may be asked whether they "strongly agree," "agree," are "uncertain," "disagree" or "strongly disagree" with some statement. Clearly, there is an order in these categories from strong positive to strong negative views. In ordinal measurement, each category can be compared to the others in relative terms. Differences can be expressed in terms of "more" or "less," which, though still limited, is an improvement over nominal measurement.