Violations of the local independence assumption in categorization

The Threshold Theory of categorization posits that prior to
making a categorization decision a respondent assesses the
similarity between item and category and compares it against
a personal threshold. Only if the item-category similarity
exceeds this threshold should the respondent endorse the item
as a category member. The Threshold Theory thus assumes
that a single latent variable, item-category similarity, suffices
to explain all categorization patterns. We put this assumption,
known as local independence, to test by providing
respondents with sets of four items to categorize. These items
are equated in terms of their similarity to the target category,
but are by design comprised of two pairs of similar items,
such as rollerskates-skateboard and horse-mule for the
category of VEHICLES. Contrary to the local independence
assumption, the items within the pairs are more likely to
receive the same categorization decision than the items across
pairs. We explain how these categorization patterns can be
accommodated within the Threshold Theory framework,
either by giving up the assumption that everyone assesses
item-category similarity in the same way for differently
weighted category information, or by explicitly incorporating
item-item similarity in addition to item-category similarity.