“The process of psychotherapy represents anything that can be conceptualized as a constituent of the psychotherapeutic treatment; more specifically, it refers to all of the events that, during the course of a treatment, occur as part of the therapy sessions and/or may be related to these sessions (Hill and Lambert 2004 ; Orlinsky et al. 2004 ). These events may refer to any domain (physiological, affective, cognitive, behavioral, etc.) that is ascribable to the client, to the therapist, and/or to their relationship. This definition is highly inclusive and, thus, analogous to the definition of the treatment process that is given by Orlinsky et al (2004 ); therefore, it should be distinguished from the more specific change process, which refers to those specific aspects of the treatment process that represent any clinically meaningful event “through which clients or patients are hypothesized to improve”(p. 312; also see Greenberg 1986 ; Rice and Greenberg 1984 ).”
“The outcome of psychotherapy represents anything that can be conceptualized as a clinical effect of the psychotherapeutic treatment process; more specifically, it refers to those clinically meaningful changes that, during the course of a treatment and/or after its completion, may be observed as results of the therapeutic process, as defined above. These changes refer to the client’s problematic domains (e.g., physiological, affective, cognitive, or behavioral) that represent the treatment’s target, and should be observed outside the treatment situation (Orlinsky et al. 2004 ).”
Gelo, O. C. G., & Manzo, S. (2015). Quantitative approaches to treatment process, change process, and process-outcome research. In Psychotherapy Research (pp. 247-277). Springer Vienna.