Nominal number

Nominal numbers are numerals used as labels to identify items uniquely. Importantly, the actual values of the numbers which these numerals represent are less relevant, as they do not indicate quantity, rank, or any other measurement.

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Labelling referees Smith and Kumar as referees "1" and "2" is a use of nominal numbers. Any set of numbers (a subset of the natural numbers) will be consistent labels as long as a distinct number is uniquely used for each distinct term which needs to be labelled. Nonetheless, sequences of integers may naturally be used as the simplest way to begin labelling; for example, 1, 2, 3, and so on.

The term "nominal number" may be quite recent and of limited use. It appears^{[citation needed]} to have originated in school textbooks derived from the statistical term "nominal data", defined as data indicating "...merely statements of qualitative category of membership." This usage comes from the sense of nominal as "name".

Mathematically, nominal numbering is a one-to-one and onto function from a set of objects being named to a set of numerals, which may change (typically growing) over time: it is a function because each object is assigned a single numeral, it is one-to-one because different objects are assigned different numerals, and it is onto because every numeral in the set at a given time has associated with it a single named object.

"Nominal number" can be broadly defined as "any numeral used for identification, however it was assigned", or narrowly as "a numeral with no information other than identification".

For the purposes of naming, the term "number" is often used loosely to refer to any string (sequence of symbols), which may not consist entirely of digits—it is often alphanumeric. For instance, UK National Insurance numbers, some driver's licence numbers, and some serial numbers contain letters.

Use of nominal numbers

"Nominal" refers to the use of numbers: any nominal number can be used by its numerical value as an integer—added to another, multiplied, compared in magnitude, and so forth—but for nominal numbers these operations are not, in general, meaningful. For example, the ZIP code 11111 is less than the ZIP code 12345, but that does not necessarily mean that 11111 was issued before 12345 or that the region denoted by 11111 is further south than 12345, though it might be. Similarly, one can add or subtract ZIP codes, but this is meaningless: 12345 − 11111 does not have any meaning as a ZIP code.

In general, the only meaningful operation with nominal numbers is to compare two nominal numbers to see whether they are identical or not (whether they refer to the same object).

A great variety of numbers meet the broad definition, including:

• National identification numbers, such as:
  • Social Security numbers
  • Driver's license numbers
  • National Insurance number
• Routing numbers, such as:
  • Bank codes and sort codes, such as International Bank Account Numbers or ABA routing transit numbers.
  • Postal codes, such as ZIP codes (These are generally numeric in the United States, but other nations often use alphanumeric systems.)
  • Telephone numbers, assigned by various telephone numbering plans, such as the ITU-T E.164 and the North American Numbering Plan (NANPA).
  • Numbers of train or bus routes or the individual vehicles in public transport
• Car model names from some car manufacturers, such as BMW or Peugeot, are plain numbers.

These are usually assigned either in some hierarchical way, such as how telephone numbers are assigned (in NANPA) as Country Code + Area Code + Prefix + Suffix, where the first three are geographically based, or sequentially, as in serial numbers; these latter are thus properly ordinal numbers.

• Numbering scheme
• Serial number
• Symbol (programming)
• Unique key
• Universally Unique Identifier

• Cardinal, Ordinal and Nominal Numbers
• Cardinal, Ordinal, and Nominal Numbers
• Posamentier, Alfred S.; Farber, William; Germain-Williams, Terri L.; Paris, Elaine; Thaller, Bernd; Lehmann, Ingmar (2013). "Nominal Numbers". 100 Commonly Asked Questions in Math Class. Corwin Press. p. 60. ISBN 978-1-4833-3399-1.