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Nominal numbers or categorical numbers are numerals used for identification only. The numerical value is irrelevant, and they do not indicate quantity, rank, or any other measurement. ==Definition== The term "nominal number" is quite recent and of limited use. It appears to have originated as a usage 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, National Insurance numbers, some driver's license numbers, and some serial numbers contain letters. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「nominal number」の詳細全文を読む スポンサード リンク
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