Liam Magee, in Towards a Semantic Web, 2011 Theories of categorisation However, proponents argue that this model is more reflective of real-world categories in psychopathology (Chapman & Chapman, 1969). It is likely that the adoption of this model would result in a decrease in reliability compared to the DSM. Furthermore, categories in the prototype model have indistinct boundaries, and the membership decision relies largely on clinician judgment. In a prototype model the level of membership is correlated with number of features present, and features are neither necessary nor sufficient since membership is not an absolute. Prototype models focus on definition by example, polythetic models focus on category membership as achieved by the presence of certain features that are sufficient. Prototype and polythetic models allow variability among features within a category, however, they view category definition differently. Although the DSM does use polythetic definitions, it does not constitute a prototypical model because specific subsets of symptoms are sufficient for making a diagnosis. Some authors have described the DSM system as a prototype model, primarily because it uses polythetic, as opposed to monothetic, definitions ( Clarkin et al., 1983 Widiger & Frances, 1985). The prototype model suggests that the degree of membership to a category is correlated with the number of features that a member has, so defining features are neither necessary nor sufficient. Rather, the prototype model holds that membership in a category is correlated with the number of representative symptoms the patient has. The categories are defined by exemplars, or prototypes, and the presentation of features or symptoms in an individual is neither necessary nor sufficient to determine membership in a category. Diagnostic groups are not viewed as discrete, but individuals may warrant membership in a category to a greater or lesser degree. First, the prototype model is based on a philosophy of nominalism, in which diagnostic categories represent concepts used by mental health professionals ( Blashfield, 1991). The prototype model differs from the Neo-Kraepelinian model in several ways. A prototype consists of the most common features of members of a category, and is the standard against which patients are evaluated ( Horowitz et al., 1981). In this system, patients' symptoms are evaluated in terms of their correlation with a standard or prototypical representation of specific disorders. The prototype model has been suggested as a viable alternative to the current Neo-Kraepelinian approach (Cantor, Smith, French, & Mezzich, 1980 Clarkin, Widiger, Frances, Hurt, & Gilmore, 1983 Horowitz, Post, French, Wallis, & Siegelman, 1981 Livesley, 1985a, 1985b). Feuer, in Comprehensive Clinical Psychology, 1998 3.08.5.2.2 Prototype model
This method is formally identical to assessing the similarity of an item to the prototype using an exponentially decaying squared-distance function, as described above. The similarity-based prototype models can be mathematically formalized by specifying a Gaussian distribution over feature values for each category, with the mean and variance of each distribution depending on the category. To model the prototype view with the Bayesian network model of Figure 6.1, the C variable would take on states representing different possible categories, and the feature values would depend probabilistically on which category the item is a member of ( Figure 6.2).
Some prototype models, for example, assume that the similarity of an item to the prototype of a category is an exponentially decaying function of the psychological distance (or squared distance) between them ( Nosofsky, 1987, 1992). Categorization is then a function of how similar the category’s prototype is to the object to be categorized. Prototype models assume that for each category people retain in memory a single specific example (the prototype), and that category members in the world fall in a distribution around the prototype ( Fried & Holyoak, 1984 Reed, 1972). Krynski, in Psychology of Learning and Motivation, 2013 2.1.1 Prototype Models