Investigating the evolution of the self in psychotherapy with a mixed-method approach: A pilot study

Objective: This pilot study aimed to assess the feasibility and informativeness of a mixed-method approach to investigate how the Self changes during psychotherapy within a Dynamic Systems Theory (DST) framework, addressing theoretical heterogeneity and the limits of conventional pre–post designs. Method: Five participants undergoing psychotherapy were scheduled to complete weekly smartphone-based diary assessments over several months (≥30 scheduled prompts). Each assessment included 11 (nomothetic) ad hoc items and 2 person-specific (idiographic) items. Responses to the Narrative Assessment Interview (NAI) were analyzed using conventional qualitative content analysis to derive person-specific items and interpret quantitative trajectories (but were not analyzed as a separate qualitative outcome). Quantitative data were analyzed using time-series methods (i.e., trend analysis and ARIMA modeling). Results: The Self exhibited various change patterns (e.g., linear, nonlinear, stationary, and non-stationary), characterized by the influence of past values (i.e., the autoregressive component) and innovations (the moving-average component). Moreover, participants exhibited different change dynamics. Therefore, the conditions required for ergodic generalization were often not met in these data. Conclusion: The mixed-methods approach was informative, capturing the complexity of Self-change. The method was feasible in a real clinical setting, but compliance and the time required to perform the analyses were critical. © 2026 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.