Statistical analysis plan for a cluster randomised controlled trial to compare screening, feedback and intervention for child anxiety problems to usual school practice: identifying Child Anxiety Through Schools-identification to intervention (iCATS-i2i).

"Declarations Ethics approval and consent to participateThe study has been approved by the University of Oxford’s Medical Sciences Interdivisional Research Ethics Committee (Reference: R66068). Approval will be sought from the same ethics committee for any amendments to the protocol. Schools’ headteachers provide written agreement (online or on paper). Parents have the opportunity to opt their child out of the study. Written assent/consent is obtained from child/adult participants respectively. Opt-in parental consent is required for child qualitative interviews. Explicit child assent, and consent from parents, school staff and those involved in delivering the intervention, is obtained for audio recording of qualitative interviews. Sample consent forms are available from the corresponding author. Consent for publicationNot applicable Competing interestsCathy Creswell (C. C.) receives royalties for the book on which this intervention is based; however, none of the authors makes any financial gain from the intervention used in this study. Susan Spence (S. S.) owns the copyright for the Spence Children’s Anxiety Scale (SCAS) items but will not benefit financially from its use in this study. Tamsin Ford’s research group receives funding for research consultancy from Place2Be, a third sector organisation that provides mental health interventions and training in schools. The other authors declare that they have no competing interests. Competing interests Cathy Creswell (C. C.) receives royalties for the book on which this intervention is based; however, none of the authors makes any financial gain from the intervention used in this study. Susan Spence (S. S.) owns the copyright for the Spence Children’s Anxiety Scale (SCAS) items but will not benefit financially from its use in this study. Tamsin Ford’s research group receives funding for research consultancy from Place2Be, a third sector organisation that provides mental health interventions and training in schools. The other authors declare that they have no competing interests."

"Funding This study is funded by the National Institute for Health and Care Research (NIHR) Programme Grants for Applied Research (Reference Number: RP-PG-0218–20010). C. C. was supported by the Oxford and Thames Valley National Institute for Health Research Applied Research Collaboration. O. U. and S. B. were supported by the National Institute for Health Research Applied Research Collaboration South West Peninsula. A. G. was partly supported by the NIHR Biomedical Research Centre Oxford. M. V. was partly supported by the NIHR Oxford Health Biomedical Research Centre (BRC) and receives funding from the NIHR Applied Research Collaboration Oxford and Thames Valley at Oxford Health NHS Foundation Trust. The views expressed are those of the authors and not necessarily those of the NHS, NIHR or the Department of Health and Social Care."

"Trial registration ClinicalTrials.gov ISRCTN76119074. Registered on 4 January 2022"