Fuzzy Concepts in Psychotherapy Research

Psychotherapy research often requires concepts like depression, which refer to global subjective states that include a number of co-occurring internal events. Since these internal events vary in their salience both within and between persons, the concept itself assumes a slightly different meaning from person to person and from occasion to occasion. One usual way to define such concepts is first to describe a prototypic ideal, composed of the most frequent characteristics of members of the category, and then to specify the degree to which a particular instance approximates the ideal. Good instances approximate the ideal well, but ambiguous instances approximate it only partially. This paper describes the inevitable instability or unreliability when such concepts are used, and a procedure is proposed for identifying and removing ambiguous instances of the concept in order to increase interjudge agreement. The concept depression is examined further to clarify its origin as a fuzzy concept and to explore implications for psychotherapy. We distinguish between (a) a person's subjective experience on a specific occasion, (b) a person's representation across occasions (the “schema”), and (c) the modal schema across persons (the “prototype”).