Heil 2008.

Methods	Study design: RCT; conducted between 2001 and 2003 Country: USA Setting: 4 group obstetric practices and the Women, Infants, and Children (WIC) programme in Burlington, VT
Participants	82 women, gestational age ≤ 20 weeks, smoked at all in the previous 7 days, locally resident, English‐speaking Mean age 24, Education 11.8 years, mean CPD 18.6; mean gestation 9 weeks
Interventions	Abstinence monitoring for first 5 days (Monday to Friday) for all participants, then twice weekly (Mondays, Thursdays) for next 7 weeks, then once a week (Wednesday) for 4 weeks, then every other Wednesday until delivery. In post‐partum period, monitoring increased to every Wednesday for first 4 weeks, then bi‐weekly (every other Wednesday) for next 8 weeks to week 12. Final 24‐week post‐partum testing All participants received standard care from their clinic, over and above trial conditions. Also a pamphlet from study staff at baseline, plus another for those not smoking at end of pregnancy. NRT was discouraged for all participants, as it might contaminate testing Experimental Group (40 women): Contingent vouchers: awarded for proven abstinence during first 5 days. From 2nd week vouchers given for urine cotinine ≤ 80 ng/ml. Vouchers entirely contingent on biochemical specimens, not on self‐report. Values started at USD 6.25, increasing by USD 1.25 per consecutive negative sample, to a maximum of USD 45, where they stayed until a missed visit or a positive test. If reset required, value went back to start point, but 2 valid tests restored to previous level Control Group (37 women): Vouchers delivered independent of smoking status, at USD 15 per visit antepartum and USD 20 per visit post‐partum. This would average the mean payments earned in the other group Voucher‐based reinforcement therapy (VBRT) applied until 12 weeks post‐partum
Outcomes	Abstinence at end of pregnancy, 12 and 24 weeks post‐partum Biochemical confirmation by urine cotinine < 80 ng/ml, apart from CO ≤ 6 ppm for first week
Notes	New for 2015 update Funding was from research grants R01DA14028 (STH) and GCRC M01RR109
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "Randomization was stratified based on the clinic where participants received their pre‐natal care". Participants "were assigned randomly".
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Abstinence biochemically validated
Incomplete outcome data (attrition bias) All outcomes	Low risk	Relatively high compliance (83% to 95%) with assessment schedules, and no differences between groups. Withdrawals only for termination or foetal death. 3 intervention and 2 control participants removed from the denominators