### Types of Measurement Scales

Nominal variables allow for only qualitative classification. That is, they can be measured only in terms of whether the individual items belong to some distinctively different categories, but we cannot quantify or even rank order those categories. Typical examples of nominal variables are gender, race, color, city, etc.
Ordinal variables allow us to rank order the items we measure in terms of which has less and which has more of the quality represented by the variable, but still they do not allow us to say "how much more.” A typical example of an ordinal variable is the socioeconomic status of families. An ordinal scale is an ordered set of categories.  Ordinal measurements tell you the direction of difference between two individuals
Interval variables allow us not only to rank order the items that are measured, but also to quantify and compare the sizes of differences between them. An interval scale is an ordered series of equal-sized categories.  Interval measurements identify the direction and magnitude of a difference.  The zero point is located arbitrarily on an interval scale. For example, temperature, as measured in degrees Fahrenheit or Celsius, constitutes an interval scale.
Ratio variables are very similar to interval variables; in addition to all the properties of interval variables, they feature an identifiable absolute zero point, thus they allow for statements such as x is two times more than y. Ratio measurements identify the direction and magnitude of differences and allow ratio comparisons of measurements. Typical examples of ratio scales are measures of time or space.