Van Osch 2008

Methods	Setting: participants in national Quit and Win contest, Netherlands Recruitment: email to Quit and Win participants
Participants	1566 participants in national Quit & Win contest (daily smokers, smoking for at least 1 year, 18 years or older) 60.8% female, average age 36.2, average cpd 18.5, average length of smoking 19.1 years
Interventions	Quit and Win contest included 1‐month cessation period, including computer‐tailored cessation advice and telephone counselling Intervention: participants asked to formulate three coping plans when completing baseline survey Control: baseline survey only (not prompted to formulate coping strategies)
Outcomes	Continuous abstinence and 7‐d PP at 7 months Validation: none, although participants had buddies and were informed that biochemical abstinence would be performed for contest winners
Notes	New for 2013 update Unclear how abstinence data were obtained Including only respondents increased evidence of effect
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	"Based on odd or even registration numbers"
Allocation concealment (selection bias)	Unclear risk	Centralised, but unclear whether participants aware of their registration numbers
Blinding of participants and personnel (performance bias) All outcomes	Low risk	No blinding reported, but because of the nature of the intervention, performance bias unlikely
Blinding of outcome assessment (detection bias) All outcomes	Low risk	'Buddy' validation and knowledge of biochemical validation would be used for any contest winners, nature of intervention made differential misreport unlikely
Incomplete outcome data (attrition bias) All outcomes	High risk	Very high rates of dropout at 7 months (64% control, 63% intervention). "The relatively high attrition suffered across the two follow‐up measurements may restrict validity of the results and may have caused biases in reported abstinence rates"