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BACKGROUND: The Identifying Child Anxiety Through Schools-identification to intervention (iCATS-i2i) trial is being conducted to establish whether 'screening and intervention', consisting of usual school practice plus a pathway comprising screening, feedback and a brief parent-led online intervention (OSI: Online Support and Intervention for child anxiety), bring clinical and health economic benefits compared to usual school practice and assessment only - 'usual school practice', for children aged 8-9 years in the following: (1) the 'target population', who initially screen positive for anxiety problems according to a two-item parent-report child anxiety questionnaire - iCATS-2, and (2) the 'total population', comprising all children in participating classes. This article describes the detailed statistical analysis plan for the trial. METHODS AND DESIGN: iCATS-i2i is a definitive, superiority, pragmatic, school-based cluster randomised controlled trial (with internal pilot), with two parallel groups. Schools are randomised 1:1 to receive either screening and intervention or usual school practice. This article describes the following: trial objectives and outcomes; statistical analysis principles, including detailed estimand information necessary for aligning trial objectives, conduct, analyses and interpretation when there are different analysis populations and outcome measures to be considered; and planned main analyses, sensitivity and additional analyses. TRIAL REGISTRATION: ISRCTN76119074. Registered on 4 January 2022.

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Anxiety problems, Cluster randomised controlled trial, Estimand, School based, Screening, Statistical analysis plan, Child, Humans, Feedback, Anxiety, Schools, Anxiety Disorders, Parents