Nominal category
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A nominal category or a nominal group is a group of objects or ideas that can be collectively grouped on the basis of shared, arbitrary characteristic.
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 sequential, ordinal or ratio orders.
For example "race" is a nominal group. A person can be a member of a certain nominal racial group, such as black, white, or Asian, or he cannot. A black person does not have "more race" than a white person, therefore it is impossible to "order" races 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 any relationships that occur are usually deemed to be spurious, and thus unimportant. For example, trying to figure out how many black people have first names starting with the letter 'a' would be a fairly arbitrary, random exercise.
Nominal categories of data are thus most commonly 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). Nominal variables are seldom the variables being explained by the research.