ISRCTN69644721.

Study characteristics
Methods	Study design: RCT Study grouping: parallel group Unit of randomisation: individuals Power (power sample size calculation, level of power achieved): not specified in trial registration Imputation of missing data: not specified
Participants	Country: UK Setting: 4 Mind sites: Peterborough and Fenland, Tyneside, Wirral, or London (City, Hackney and Waltham Forest) Age: not specified Sample size (randomised): 255 (targeted) Sex: not specified Comorbidity (mean (SD) of respective measures in indicated, if available) at baseline: not specified Population description: adults aged 18 to 67 years old who work in 1 of the 4 emergency services: police, fire and rescue, ambulance, and search and rescue Inclusion criteria: 1) adults aged 18 to 67 years old; 2) fluent in English; 3) work in 1 of the 4 emergency services: police, fire and rescue, ambulance, and search and rescue Exclusion criteria: Participants who were depressed or suffering from post‐traumatic stress disorder and who required treatment for these conditions. Attrition (withdrawals and exclusions): not specified Reasons for missing data: not specified
Interventions	Intervention 1: new resilience intervention (n not specified) delivery: online (digital modules) and face‐to‐face (group sessions); individual and group setting providers: group sessions provided at local Mind centres duration of treatment period and timing: 4 weeks in total each week participant completes 1 x 15/20‐minute digital module 4 weekly 2‐hour group sessions with break description: DIGITAL MODULES: 4 digital modules covering 4 main topics linked to maintaining resilience (attention training, dwelling, dealing with difficult emotions, transforming worry) GROUP SESSIONS: cover experiential exercises, work in pairs and group discussion; cover main topics linked to maintaining resilience compliance: not specified integrity of delivery: not specified economic information: not specified theoretical basis: no theoretical foundation specified Intervention 2: digital‐only intervention (n not specified) delivery: online (reading material) providers: self‐guided duration of treatment period and timing: 4 weeks; weekly 30‐minute online modules a week description: reading material about mental health and well‐being compliance: not specified integrity of delivery: not specified economic information: not specified theoretical basis: not specified Control: wait‐list control (n not specified; received new resilience intervention 4 months later)
Outcomes	Outcomes collected and reported: Primary outcome well‐being ‐ Warwick Edinburgh Mental Wellbeing scale and ONS (Office for National Statistics) well‐being questions (item 1) mindful attention ‐ Mindful Attention and Awareness Scale Secondary outcome general health ‐ General Health Questionnaire‐12 resilience ‐ statements about resilience life satisfaction ‐ statements about life satisfaction awareness of mental health management tools ‐ questions about knowledge of mental health management tools rumination ‐ statements about dwelling depression ‐ Patient Health Questionnaire‐9 anxiety ‐ Generalized Anxiety Disorder 7 Outcomes reported not specified Time points measured and reported: 1) pre‐intervention; 2) post‐intervention; 3) 3‐month follow‐up (3 months post‐intervention); time points reported not specified Adverse events: not specified
Notes	Contact with authors: We contacted the authors for the dates the trial was completed and published and if the authors could provide the summary outcome data for the 2 groups (Wild 2018 [pers comm]). Study start/end date: October 2016 – April 2017 Funding source: University of Oxford; Mind, the mental health charity (UK) Declaration of interest: not specified Ethical approval needed/obtained for study: approved by Medical Sciences Inter‐Divisional Research Ethics Committee, 14 October 2016, ref: R47862/RE001 Comments by authors: not specified Miscellaneous outcomes by the review authors: information received from authors (Wild 2018 [pers comm]): trial completed but unpublished; study conducted at 4 Mind centres in Peterborough and Fenland, Tyneside, Wirral, or London Correspondence: Dr Jennifer Wild (primary contact), Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford, OX1 3UD, United Kingdom; Jennifer.wild@psy.ox.ac.uk
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote (see trial registration): "Participants are then randomly allocated to one of three groups." Quote (see trial registration): "Emergency workers will be randomly allocated to receive one of the following three interventions: 1. The new resilience intervention (...), 2. The digital‐only intervention (...), 3. The wait‐list condition (...)" Judgement comment: based on trial registration, insufficient information about random‐sequence generation to permit judgement of 'Low risk' or 'High risk'; no judgement on baseline comparability in sociodemographic and outcome variables possible based on trial registration
Allocation concealment (selection bias)	Unclear risk	Judgement comment: based on trial registration, insufficient information about allocation concealment to permit judgement of 'Low risk' or 'High risk'
Blinding of participants and personnel (performance bias) Subjective outcomes	High risk	Judgement comment: based on trial registration, blinding of participants and personnel probably not done (1 group includes face‐to‐face group sessions) and the outcome is likely to be influenced by lack of blinding
Blinding of outcome assessment (detection bias) Subjective outcomes	High risk	Judgement comment: based on trial registration, insufficient information about blinding of outcome assessment; however, due to potential performance bias (no blinding of participants), the review authors judge that the participants' responses to questionnaires may be affected by the lack of blinding (i.e. knowledge and beliefs about intervention they received)
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Judgement comment: no judgement possible based on trial registration
Selective reporting (reporting bias)	Unclear risk	Judgement comment: no judgement possible based on trial registration