6 Sample size determination: Since this is a pilot study to test the feasibility and generate preliminary estimates of effectiveness of the interactive messaging service for smoking cessation, the sample size would be 50 for each arm (i.e. total sample size = 100). Taking reference from the relapse rate (based on the 7-day point prevalence) of ICSCs of TWGH from 2009-2010, 251 out of the 763 quitters at 8 weeks reported that they smoked again at 6 months. In other words, the relapse rate (number of continuing smoking at 6 months among self-reported quitters at the 8-week follow-up) was 33%, assuming no new quitters. We assumed a similar result would occur for the control group of this RCT of relapse prevention (Group B). The effect size is set conservatively at 1:1.5 for the contrast between group A and B. In other words, the estimated relapse rates for A and B are 22% and 33%, respectively. As we only have 100 subjects in total, the power for detecting this difference using the Fisher's exact test is 26%, given alpha is 0.05. That means the study has low probability to reject the alternative hypothesis if it is false. The power would reach to 51.4% when the estimated effect size is set at 1:2. Alternatively, if the total sample size can be increased to 500, the power can reach to 85% for the 1.5 effect size.