Measures
At 6 weeks, participants were sent a text message with a questionnaire link, 1 day after receiving a prenotification text message alerting them to the questionnaire link message and a texted £5 Amazon voucher for completing it. A reminder text message was sent if the survey was not completed 4 days later, after which participants were informed a researcher would call to complete it over the phone. The researcher made up to five contact attempt episodes.Six and a half months after enrollment (“6-month follow-up”), the same procedure as for the 6-week questionnaire was undertaken for the 6-month questionnaire, with participants randomized to receive either a £10 or £20 Amazon voucher incentive for completion, as part of the SWAT (with trial arm included as a stratifier to ensure the balance between incentive groups), reported elsewhere. If manual follow-up by telephone was unsuccessful, participants were sent a text inviting a response to the primary smoking outcome question. Participants reporting 7-day abstinence at 6 months were sent a postal saliva test kit and texted a £5 voucher incentive by the lead researcher if returned to the testing laboratory.