Inclusion and exclusion criteria and underlying rationale
| Inclusion criteria | Rationale |
| (1) Informed consent via the web form | To ensure knowledge of procedures and the declaration of consent |
| (2) Minimal age of 18 years | To ensure a minimum age of participation |
| (3) Gambling activity at least once weekly over the 30 days prior to study entry | To ensure that the programme reaches the intended population; increase validity |
| (4) A PGSI score ≥3 | To ensure that the programme reaches the intended population; increase validity |
| (5) At least once weekly internet access and a valid email address | To ensure at least some access to the intervention |
| (6) Proficiency in German or French | To ensure that participants will be able to understand the information provided |
| Exclusion criteria | Rationale |
| (1) Self-reported engagement in other outside psychosocial treatments for problem gambling | To avoid confounding treatment effects |
| (2) Last 90-day psychosis or mania | To avoid having individuals with these problems enter the study |
| (3) Presence of a severe substance use disorder (DAST score >5 or AUDIT score >20) | Individuals with a severe substance use disorder need more intensive treatment than provided |
| (4) Elevated suicidality (scoring greater than ‘minimal risk’ on a screener) | To increase safety, participants with elevated suicidality will be referred for face-to-face treatment |
AUDIT, Alcohol Use Disorders Identification Test; DAST, Drug Abuse Screening Test; PGSI, Problem Gambling Severity Index.