The collection of data requires structure, including an experimental design and a method of observation. A nominal scale is a qualitative categorization according to unordered distinctions. Some academic institutions, such as the Massachusetts Institute of Technology, have assigned a number to every department. An ordinal scale makes ranked distinctions. It depends on only one property, the sequence of letters in the word.
Military rank is another example of an ordinal scale.
An interval scale is based on the real numbers, so that each unit on the scale expresses the same degree of difference, no matter where on the scale it is located. For instance, weight, length, and duration of time are interval scales. The atomic number of the elements of chemistry is regarded as an interval scale, even though the realizable values are all integers. The difference it expresses between elements one apart is always one proton in the nucleus. If the value of zero on an interval scale represents a total absence of the property being measured, then the scale is sometimes called a ratio scale .