Fig. 1

Study design for the pilot study. Illustrating the time points of the interview and cotinine urine tests plus the amount of money the participants could receive at each time point. The green cotinine urine tests represent negative tests indicating no exposure to smoking. The Figure illustrates the amount of money the vouchers would represent if a participant would be present at the interview and consecutively would have four negative cotinine urine tests indicating she had stopped smoking. In case a participant would have a positive cotinine urine test in week 7, this would result in no voucher at that appointment and consequently a final voucher of €150 euros in week 10 if the cotinine urine test would then again be negative