Sixty undergraduate students made category membership decisions for each of 132
candidate exemplar-category name pairs (e.g., chess – Sports) in each of two separate
sessions. They were frequently inconsistent from one session to the next, both for nominal
categories such as Sports and Fish, and ad hoc categories such as Things You Rescue from
a Burning House. A mixture model analysis revealed that several of these inconsistencies
could be attributed to criterial vagueness: participants adopting different criteria for
membership in the two sessions. This finding indicates that categorization is a probabilistic
process, whereby the conditions for applying a category label are not invariant. Individuals
have various functional meanings of nominal categories at their disposal and entertain
competing goals for ad hoc categories