Table 1

Characteristics of previous categorisation attempts

Classification	Strengths	Issues
Common Language for Psychotherapy Procedures (CLP) group, 2010–2018.16–19
Cognitive Behavioural Emotional Somatic Cognitive–behavioural Behavioural–emotional Unclassified	Systematic approach to categorisation/classification. Internal consistency tested between separate teams of authors. Based on sample of interventions. eBook provides supplementary source of information on selected psychotherapy interventions.	Preliminary classification: sample limited to only those interventions for which descriptions had been submitted (passive sampling). Does not accommodate all interventions found in Cochrane register. Classification divided by goals of therapy rather than intended mechanism. Less applicable for research/systematic review purposes. Less easily used by lay-person. Less congruent with existing classification systems for pharmacological therapies.
Hersen and Sledge’s Encylopaedia of Psychotherapy, 2002. 8
N/A	Comprehensive source of information on selected psychotherapy interventions.	No formal classification of psychotherapy approaches provided.
Kazdin’s Encyclopaedia of Psychology, 2000. 9
Classification 1* Eclectic Cognitive Psychoanalytic/dynamic Interpersonal Existential/humanistic Systems Classification 2† Psychoanalytic/dynamic Behavioural Cognitive Humanistic Integrative/eclectic	Categorisation by theoretical underpinning—practically similar to categorisation by mechanism. Recognisable and commonly used terminology used for categories. Consistency of categorisation between chapter authors. Classification 1 is based on a survey of psychologists’ self-identified school of psychotherapy.	Not based on a defined sample of interventions. Largely non-systematic approach, method for creation of classification 2 not stated. Integrative/eclectic category not practical for research/systematic review purposes. Classification 1 lacks a category adequately describing behavioural therapies. Classification 2 lacks a category adequately describing social/family/interpersonal therapies.
The Multitheoretical List of Therapeutic Interventions (MULTI)‡, 2009–2018. 10 20
Behavioural Cognitive Dialectical-behavioural Interpersonal Person-centred Psychodynamic Process-experiential Common factors	Designed to be usable by lay-persons as well as professionals. Developed from wide range of sources and experts, with extensive testing and refinement. Internal consistency tested between patients, therapists and observers. Mechanism-based categorisation, most consistent with our own conception of a psychotherapy classification.	Format is a questionnaire completed by therapists/patients/observers, rather than a simple description of categories. Hence not practical for use in efficient categorisation of studies. Definitions of each category not provided. Unclear what the specific characteristics of less commonly recognised categories are. Designed for application to clinical practice as opposed to research.

*From Chapter: ‘Psychotherapy: Clinical Practice’.21
†From Chapter: ‘Psychotherapy: Approaches’.22
‡Papers describing the MULTI were identified subsequent to the generation of our own classification, and hence were not primary sources for our classification. Nevertheless after the MULTI was encountered it was found to have a significant degree of consistency with our own classification. The ‘subscales’ of ‘Person Centred’, ‘Common Factors’, and ‘Dialectical-Behavioural’ broadly overlap with our “bough” of ‘Humanistic’, while the subscale of ‘Interpersonal’ corresponds to our bough of ‘Social’. After analysis of both systems, we decided that consequent changes to our own classification (such as the subdivision of the ‘Humanistic’ bough) were not immediately necessary, as they would introduce a level of complexity that would not be practical for the purposes of categorising existing studies.