NCT03713398.

Study characteristics
Methods	Open‐label, parallel, randomised clinical trial
Participants	Male and female prison inmates, 18 years and older, with moderate to high risk levels of criminogenic risk factors and a diagnosis of schizophrenia, schizoaffective disorder, psychotic disorder, bipolar disorder with psychotic features or major depressive disorder with psychotic features
Interventions	1. T4C 2. Standard prison mental health services Intervention will be delivered over a 3‐month period.
Outcomes	Primary outcomes: Aggression (12‐item Aggression Questionnaire‐Short Form) Behavioural infractions (number) Administrative segregation (times placed) Secondary outcomes: Interpersonal problem‐solving (52‐item Social Problem‐Solving Inventory) Criminal attitudes (46‐item Measure of Criminal Attitudes and Associates Part B) Impulsivity (30‐item Barratt Impulsiveness Scale)
Notes	ClinicalTrials.gov Identifier: NCT03713398 Funding: R34MH111855 (U.S. NIH Grant/Contract) Declaration of interest: none Contact of authors: PI was contacted on issues regarding administration of the experimental intervention and risk of bias.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	The author gave us the following explanation on randomisation: "In this study randomization will take place on‐site at the prison, which does not allow electronic equipment to be brought into facilities. Therefore, the study team will use shuffled envelopes to randomize participants on‐site at the prison. The study team will use a table of computer‐generated random numbers and block randomization procedures to ensure an equal distribution of subjects to each arm of the study".
Allocation concealment (selection bias)	Low risk	Use of shuffled envelopes
Blinding of participants and personnel (performance bias) All outcomes	High risk	Open‐label trial; masking was not possible.
Blinding of outcome assessment (detection bias) All outcomes	High risk	Outcome assessments are based on unblinded participant reported outcomes.
Incomplete outcome data (attrition bias) All outcomes	High risk	High level of attrition in both treatment arms. More than 50% of participants who started the trial did not complete it.
Selective reporting (reporting bias)	Low risk	All registered outcomes are reported.
Other bias	Low risk	None found

DSM‐IV: Diagnostic and Statistical Manual of Mental Disorders, fourth edition; ICD: International Classification of Diseases; R&R: reasoning and rehabilitation; T4C: Thinking for a Change; TAU: treatment as usual.