A nominal category or a nominal group is a group of objects or ideas that can be collectively grouped on the basis of a particular characteristic—a
qualitative property
Qualitative properties are properties that are observed and can generally not be measured with a numerical result. They are contrasted to quantitative properties which have numerical characteristics.
Some engineering and scientific properties are ...
.
[.] A variable that codes whether each one in a set of observations is in a particular nominal category is called a
categorical variable
In statistics, a categorical variable (also called qualitative variable) is a variable that can take on one of a limited, and usually fixed, number of possible values, assigning each individual or other unit of observation to a particular group or ...
.
Valid data operations
A nominal group only has members and non-members. That is, nothing more can be said about the members of the group other than they are part of the group.
Nominal categories cannot be numerically organized or ranked. The members of a nominal group cannot be placed in
ordinal (sequential) or
ratio
In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
form.
Nominal categories of data are often compared to ordinal and ratio data, to see if nominal categories play a role in determining these other factors. For example, the effect of race (nominal) on income (ratio) could be investigated by
regressing the level of income upon one or more
dummy variables that specify race. When nominal variables are to be explained,
logistic regression
In statistics, the logistic model (or logit model) is a statistical model that models the probability of an event taking place by having the log-odds for the event be a linear combination of one or more independent variables. In regression an ...
or
probit regression
In statistics, a probit model is a type of regression where the dependent variable can take only two values, for example married or not married. The word is a portmanteau, coming from ''probability'' + ''unit''. The purpose of the model is to es ...
is commonly used.
Examples
For example, citizenship is a nominal group.
A person can either be a citizen of a country or not. One citizen of Canada does not have "more citizenship" than another citizen of Canada; therefore it is impossible to order citizenship according to any sort of mathematical logic.
Another example would be "words that start with the letter 'a'". There are thousands of words that start with the letter 'a' but none have "more" of this nominal quality than others.
Correlating two nominal categories is thus very difficult, because some relationships that occur are actually
spurious
Spurious may refer to:
* Spurious relationship in statistics
* Spurious emission or spurious tone in radio engineering
* Spurious key in cryptography
* Spurious interrupt in computing
* Spurious wakeup in computing
* ''Spurious'', a 2011 novel b ...
, and thus unimportant. For example, trying to figure out whether proportionally more Canadians have first names starting with the letter 'a' than non-Canadians would be a fairly arbitrary, random exercise.
See also
*
Level of measurement
Level of measurement or scale of measure is a classification that describes the nature of information within the values assigned to variables. Psychologist Stanley Smith Stevens developed the best-known classification with four levels, or scal ...
References
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Statistical data types
Categorical data