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)

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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), brings clinical and health economic benefits compared to usual school practice and assessment only – ‘usual school practice’, for children aged 8-9 years in (1) the ‘target population’, who initially screen positive for anxiety problems according to a 2-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: 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; planned main analyses, sensitivity and additional analyses. Trial registration: ISRCTN registry ISRCTN76119074. Registered on 4 January 2022.