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Background: Internet-based cognitive-behavioural therapy (iCBT) is effective for subthreshold depression. However, which skills provided in iCBT packages are more effective than others is unclear. Such knowledge can inform construction of more effective and efficient iCBT programmes., Objective: To examine the efficacy of five components of iCBT for subthreshold depression., Methods: We conducted an factorial trial using a smartphone app, randomly allocating presence or absence of five iCBT skills including self-monitoring, behavioural activation (BA), cognitive restructuring (CR), assertiveness training (AT) and problem-solving. Participants were university students with subthreshold depression. The primary outcome was the change on the Patient Health Questionnaire-9 (PHQ-9) from baseline to week 8. Secondary outcomes included changes in CBT skills., Findings: We randomised a total of 1093 participants. In all groups, participants had a significant PHQ-9 reduction from baseline to week 8. Depression reduction was not significantly different between presence or absence of any component, with corresponding standardised mean differences (negative values indicate specific efficacy in favour of the component) ranging between -0.04 (95% CI -0.16 to 0.08) for BA and 0.06 (95% CI -0.06 to 0.18) for AT. Specific CBT skill improvements were noted for CR and AT but not for the others., Conclusions: There was significant reduction in depression for all participants regardless of the presence and absence of the examined iCBT components., Clinical Implication: We cannot yet make evidence-based recommendations for specific iCBT components. We suggest that future iCBT optimisation research should scrutinise the amount and structure of components to examine., Trial Registration Number: UMINCTR-000031307., Competing Interests: Competing interests: MS reports personal fees from SONY outside the submitted work. TAF reports grants and personal fees from Mitsubishi-Tanabe, personal fees from SONY, and personal fees from Shionogi, outside the submitted work; In addition, TAF has a patent 2020-548587 concerning smartphone CBT apps pending, and intellectual properties for Kokoro-app licensed to Mitsubishi-Tanabe. AT has received lecture fees from Dainippon-Sumitomo, Eisai, Janssen, Meiji, Mitsubishi-Tanabe, Otsuka, and Takeda. Nao Shiraishi received lecture fees from Mochida Pharma outside the submitted work. Takashi Muto received lecture fees from Aikomi and LinkMedical for work outside the submitted work. TS is employed by the Kyoto University Health Service and Health Tech Laboratory Joint Research Fund. TU received personal fees from Dainippon-Sumitomo and Shionogi Pharma outside the submitted work. Hisashi Noma reports personal fees from Boehringer Ingelheim, Kyowa Kirin, Toyota Motor Corporation, GlaxoSmithKline, Ono Pharmaceutical, SONY, and Terumo outside the submitted work. All the other authors have no competing interests to report., (© Author(s) (or their employer(s)) 2022. Re-use permitted under CC BY. Published by BMJ.)

Background: There are many different skill components used in cognitive behavioural therapy (CBT). However, there is currently no comprehensive way of measuring these skills in patients., Objective: To develop a comprehensive and brief measure of five main CBT skills: self-monitoring, behavioural activation, cognitive restructuring, assertiveness training and problem-solving., Methods: University students (N=847) who participated in a fully factorial randomised controlled trial of smartphone CBT were assessed with the CBT Skills Scale, the Patient Health Questionnaire-9 (PHQ-9), the Generalised Anxiety Disorder-7 (GAD-7) and the short form of the Japanese Big Five Scale. Structural validity was estimated with exploratory factor analysis (EFA) and confirmatory factor analysis (CFA), and internal consistency evaluated with Cronbach's α coefficients. Construct validity was evaluated with the correlations between each factor of the CBT Skills Scale, the PHQ-9, the GAD-7 and the Big Five Scale., Findings: The EFA supported a five-factor solution based on the original instruments assessing each CBT skill component. The CFA showed sufficient goodness-of-fit indices for the five-factor structure. The Cronbach's α of each factor was 0.75-0.81. Each CBT skills factor was specifically correlated to the PHQ-9, GAD-7, and the Big Five Scale., Conclusions: The CBT Skills Scale has a stable structural validity and internal consistency with a five-factor solution and appropriate content validity concerning the relationship with depression, anxiety and personality., Clinical Implications: The CBT Skills Scale will be potential predictor and effect modifier in studying the optimisation of CBT interventions., Trial Registration: CTR-000031307., Competing Interests: Competing interests: TAF reports personal fees from Mitsubishi-Tanabe, MSD and Shionogi, a grant from Mitsubishi-Tanabe, and, outside the submitted work; TAF has a patent 2018-177688 pending and intellectual properties for Kokoro-app licensed. NS received lecture fees from Dainippon-Sumitomo and Meiji-seika Pharma for work that was not associated with this article., (© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)