Hennrikus 2002.

Methods	Cluster‐randomised controlled trial Country: USA Setting: 24 Minneapolis‐St Paul worksites; 2 x 3 factorial design, stratified by gender and education
Participants	2402 current smokers (smoked at least 100 cigs in lifetime). Mean age 39, 56.2% women, 62% married/partner
Interventions	1. Group: 13 group sessions over 2 months. 2. Phone: sent printed materials, inc ALA Freedom from Smoking + 3 to 6 telephone counselling sessions 3. Choice: free choice between group or phone programmes. All programmes offered 3 times over 18 months; smokers could join more than once. Half of the sites in each intervention were offered direct incentives for participation and for quitting: Quitters at 1 month won USD 20 and entered lottery for grand prize (USD 500 as 1 prize (5 sites), 2 x USD 250 (6 sites) or 4 x USD 125 (1 site)). Drawn about every 6 months
Outcomes	Baseline, 7‐day PPA at 12 months, 24 months Validation: self‐report, countersigned by friend or family member for monthly abstinence. Grand draw prize winners + 24 months random sample of quitters (paid USD 25 for compliance) tested for salivary cotinine
Notes	Study was funded by the National Heart, Lung, and Blood Institute. Data extrapolated from percentages
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "Four worksites were randomly assigned to each of the 6 experimental conditions". Method not stated
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	High risk	Not all participants had abstinence biochemically validated
Incomplete outcome data (attrition bias) All outcomes	Low risk	At 12 months 14.5% lost, and at 24 months 18.3% lost
Other bias	High risk	Group dropouts were not followed up; phone dropouts were rung for up to 10 times for each counselling session, and were then left messages or sent letters