The purpose of the study is to investigate the process of symptom change in Cognitive Behavioural Analysis System of Psychotherapy (CBASP), and how the individual components of the therapy affect this change in individuals with chronic depression.

A form of psychotherapy called Behaviour Analysis System of Psychotherapy (CBASP) has been developed specifically to treat individuals suffering from chronic, long-standing depression. There is a growing evidence base to suggest that CBASP is effective for these individuals, especially when combined with anti-depressant medication. This evidence comes from case series and randomised controlled trials that have compared CBASP with other forms of therapy such as Interpersonal Psychotherapy and Cognitive Behaviour Therapy, and with medication. The evidence from these studies consistently shows CBASP to be effective, and it compares favourably with other forms of therapy.

Given these findings, it is important to understand how change happens in CBASP and how it is affected by the components of the therapy. Understanding how individuals experience change within CBASP will have important implications for how the therapy is delivered in the future. For example if change is found to be non-linear it would be important to understand what components of the therapy are associated with improvement. There could also be organisational implications, for example relating to the optimal number of sessions to offer in order to achieve acceptable change while balancing therapist time.

The proposed study aims to investigate the process of psychological change in CBASP. This will be done by providing CBASP to a small number of individuals and gathering outcome data (rating of mood) at every therapy appointment. Session--by-session change will then be mapped for each participant. The study will include two arms: CBASP; and CBASP without Disciplined Personal Involvement (DPI). DPI involves specifically using the therapist--client relationship to help the client to discriminate between previous maladaptive relationships and other, positive ones. The reason for including this condition is to investigate whether DPI affects the process of psychological change in CBASP, as it is a unique feature of the therapy.

The study will use a single-case design, as the aim is to map change within each individual participant in the context of the therapy. This design was deemed appropriate as the study does not aim to make claims about the overall effectiveness of CBASP, but to provide an analysis of individual psychological change for participants receiving CBASP.

Inclusion Criteria:

• Aged between 18 and 64 years, inclusive
• Have been depressed for 2 years or longer
• Have had previous episodes of depression
• Previous treatment (medication, psychotherapy or both) unsuccessful or participant has relapsed
• Do not meet any of the exclusion criteria
• Able to provide informed consent to participate in the study

Exclusion Criteria:

• Received psychological therapy in previous 12 months
• Current significant substance misuse
• Presence of learning difficulties
• Presence of psychosis
• Unable to commit to the full duration of the study