Study	Selection bias	Study design	Confounders	Blinding	Data collection	Withdrawals and drop out	Global rating	Notes
Armstrong et al. (2020)	3	1	1	1	1	1	moderate	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, fully blinded, standardized psychometric measures were used, no attrition as single-session intervention.
Bőthe et al. (2021)	2	1	1	3	1	3	weak	The selected individuals are likely to be representative of the target population, randomized controlled trial, no confounders, not blinded, standardized psychometric measures were used, less than 60% completion rate.
Bücker et al. (2018)	3	1	1	3	1	3	weak	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, no blinding, standardized psychometric measures were used, less than 60% completion rate
Bücker et al. (2021)	3	1	1	3	1	3	weak	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, no blinding, standardized psychometric measures were used, less than 60% completion rate
Caillon et al. (2019)	3	1	1	3	1	1	weak	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, not blinded, standardized psychometric measures were used, no attrition.
Caillon et al. (2021)	3	1	1	1	1	1	moderate	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, not blinded, standardized psychometric measures were used, no attrition.
Carlbring and Smit (2008)	1	1	2	NA	2	2	strong	Sample is likely to be representative (recruitment through advertisements and newspapers). Randomized controlled trial. Standardized psychometric measures were used; however, data is self-reported. 69% completed the 36-month follow-up.
Casey et al. (2017)	2	1	2	3	1	3	weak	Participants on the WLC, received the treatment before follow-up data were collected, thus there were no comparisons between the WLC, and the active treatment conditions at follow-up. Participants and project staff were not blind to allocation. Standardized valid, and reliable psychometric measures were used. 29% completed the 12-month follow-up.
Cunningham et al. (2012)	3	1	1	3	1	1	weak	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, no blinding, standardized psychometric measures were used, less than 60% completion rate
Cunningham et al. (2019a)	3	1	1	3	1	1	weak	The selected individuals are likely to be representative of the target population (recruitment through crowdsourcing website). No significant differences between intervention and control conditions, prior to the intervention were found. Blinding is not described. Follow-up: 87% at 6 weeks, 87% at 6 months.
Cunningham et al. (2019b)	1	1	1	1	2	3	moderate	The selected individuals are likely to be representative of the target population (recruitment through social media and print advertisements, across Canada). Double-blinded. Standardized, valid, and reliable psychometric measures were used, data collected by clinicians. Follow-up: 38.8% at 3-months, 34.1% 6-months.
Cunningham et al. (2020)	2	1	1	3	1	1	moderate	The selected individuals are somewhat likely to be representative of the target population (recruitment through social media). At baseline, there were no significant differences between conditions on any of the demographic variables. Follow up: 80% at 3 months, 64% at 6 months.
Dowling et al. (2021)	1	1	2	2	1	3	moderate	The selected individuals are somewhat likely to be representative of the target population, randomized controlled trial, no confounders, blinded, standardized psychometric measures were used, less than 60% completion rate.
Fitzpatrick et al. (2018)	3	1	1	3	1	3	weak	The percentage of selected participants is not described, randomized controlled trial, no confounders, not blinded, standardized psychometric measures were used, less than 60% completion rate.
He et al. (2021)	2	1	1	1	1	1	strong	The selected individuals are likely to be representative of the target population, randomized controlled trial, no confounders, fully blinded, standardized psychometric measures were used, no attrition.
Hodgins et al. (2019)	2	1	2	1	2	1	strong	The selected individuals are likely to be representative of the target population (recruitment through social media and print advertisements across Canada). There were no significant differences between conditions on any of the demographic variables. Researchers conducting follow-up were blinded to participant allocation. 66% completed all follow-ups
Jonas et al. (2020)	1	1	1	3	1	3	weak	The selected individuals are somewhat likely to be representative of the target population. (Recruitment through the website "check-dein-spiel.de"). Randomization resulted in similar groups; potential confounders were included in the analyses. Two outcomes relied on self-report. Blinding is not described. Follow-up: 53.9% at 3 months, 47.3% at 6 months and 55% ad 12 months. ITT analysis conducted.
Luquiens et al. (2016)	2	1	3	3	2	3	weak	The selected individuals are likely to be representative of the target population (recruitment directly through gambling environment). There were no exclusion criteria except for the age limit. Additional demographic characteristics or psychiatric conditions could have limited the impact of the intervention. No blinding could be applied. Attrition rate: 83%.
Muroff et al. (2010)	1	2	1	3	1	3	weak	The selected individuals are likely to be representative of the target population, quasi-experimental design, no confounders, not blinded, standardized psychometric measures were used, completion rates were not described.
Neighbors et al. (2015)	3	1	1	1	1	1	moderate	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, fully blinded, Standardized psychometric measures were used, no attrition as single-session intervention.
Nilsson et al. (2018)	2	2	3	3	1	1	weak	The selected individuals are somewhat likely to be representative of the target population (recruitment through Swedish National Gambling Helpline via an online advertisement and through health care professionals who were informed about the study).
Park et al. (2016)	3	1	1	3	1	1	weak	The percentage of selected participants is not described, randomized controlled trial, no confounders, not blinded, standardized psychometric measures were used, no attrition as single-session intervention.
Rabinovitz and Nagar (2015)	3	1	1	1	1	1	moderate	The percentage of selected participants is not described, randomized controlled trial, no confounders, not blinded, standardized psychometric measures were used, no attrition as single-session intervention.
Rodda et al. (2018)	2	1	2	2	2	3	moderate	The selected individuals are somewhat likely to be representative of the target population (recruitment through e-mental health services). There were no significant differences between conditions on any of the demographic variables, prior to treatment. Research assistants were blind to allocation. Follow-up: 4 weeks: 45%, 12-weeks: 38%
Rosen et al. (2020)	2	1	2	3	2	1	moderate	Sample may not be representative of the entire ex-offender population as some ex-offenders in these areas may not have access to the Internet or computers. It is possible that those who completed the study may already have access to more resources, which may have impacted the current results. Follow-up: 80% at 30-day follow up.
So et al. (2020)	2	1	3	1	2	1	moderate	Participants were recruited through online advertisements, however because of the incentive provided for participating, the sample may have been biased. Quadruple blinding was applied. Gambling problems of participants was based on their self-report without an in-person interview. Follow-up: 72% post-intervention assessment. Intention-to-treat analysis conducted.
Su et al. (2011)	2	2	3	NA	2	1	moderate	The selected individuals are somewhat likely to be representative of the target population (students recruited from the Beijing normal University and participants volunteered to participate). Additionally, there were more female participants (69%) than male participants. Follow-up: 91% at 1-month.
Wittekind et al. (2019)	3	1	1	1	1	3	weak	Less than 60% of the selected participants participated, randomized controlled trial, no confounders, fully blinded, Standardized psychometric measures were used, less than 60% completion rate.
Yakovenko and Hodgins (2021)	2	2	2	1	3	1	moderate	More than half of participants reported currently receiving treatment for gambling problems at baseline. Participants who are seeking treatment are more likely to be motivated to engage with interventions and to achieve their recovery goals. It is possible that some participants in the face-to-face group may have had access to the internet-based program. Interviewers were blind to participant assignment. All data came from self-report. Follow-up: 70% at 12 months.