Results
Smoking Dependence
There were 1275 study website landings from the online adverts, of which 323 (25%) people completed the eligibility assessment. Of those assessed for eligibility, 93% (299) were eligible and of those eligible, 70% (209) consented and were randomized (117 in phase one and 92 in phase two). One additional individual was randomized but had been enrolled by their partner and so was removed. See Trial flow.
Participant Characteristics at BaselineWISDM = Wisconsin Inventory of Smoking Dependence Motives.At 6 weeks, 149 (71%; 95% CI 65%, 77%) were followed up and at 6 months this was 160 (77%; 95% CI 71%, 82%). There were six withdrawals, four from the Quit Sense arm and two from the usual care arm. Completion of self-reported abstinence for the primary outcome at 6 months was 77% (160/209; 95% CI 71%, 82%)). By arm, response rates were 78% and 75% for the Quit Sense and usual care arms, respectively. The return of a viable saliva sample for biochemical validation of those self-reporting abstinence for the primary outcome was 39% (16/41; 95% CI 24%, 54%), and by arm 52% (13/25) and 19% (3/16) for Quit Sense and usual care arms respectively. At the 6-month follow-up, the response rate for both resource use and quality of life data were 147/209 (70%; 95% CI 64%, 77%).Advertising running costs were £2796 and advert costs were £804.44 for Facebook and £412.49 for Google search (grand total £4012.93). Total cost per recruit was £19.20 (US$23.80) (£13.38 running costs, £5.82 advert costs). 195 participants were recruited via Facebook and 14 from Google, with lower advert costs per recruit for Facebook (£4.13) than Google (£29.46). See The installation rate of the Quit Sense app, defined as receipt of the unique code provided to intervention participants by the app’s server, was 75% (95% CI 67%, 83%; 78/104) (Use of Quit Sense app (Quit Sense arm only)

Analysis of the primary smoking outcome, which we anticipated would be used in a future study, found a higher abstinence rate in the Quit Sense arm (11.5%; 12/104) compared to the usual care arm (2.9%; 3/105) (unadjusted OR: 4.44, 95% CI 1.21, 16.21, Between-Arm Differences in Abstinence for Smoking Outcomes




By using the Bayesian approach relevant in preliminary trials, it is estimated there is 90% probability that the underlying OR favoring the intervention is 1.7 or higher, 93% that it is 1.5 or higher, and 85% that it is 2.0 or higher, indicating good support for a subsequent trial in which this range of effect sizes is considered.Because of imbalanced saliva sample return rates between arms, we undertook a post hoc sensitivity analysis for the primary smoking outcome but using self-reported prolonged abstinence only. The findings favored the Quit Sense arm, though the between-arm difference was not statistically significant (Quit Sense 24.0%; usual care 15.2%, OR: 1.76, 95% CI 0.88, 3.53, At 6 weeks of follow-up, 70.4% in the Quit Sense arm and 80.8% in the usual care arm reported smoking in the first 2 weeks of a quit attempt (or since enrollment if no quit date was set) which was not statistically significant (