Development of the intervention framework
PMT
We based the intervention’s theoretical framework on the transtheoretical model (TTM) and protection motivation theory (PMT). For the TTM, we used several strategies to strengthen behaviour changes and/or to achieve the next stage. On the pre-quit date, we sent messages related to consciousness raising, dramatic relief, and environmental re-evaluation to smokers with weak quitting intentions, and those related to stimulus control, self-liberation, and reinforcement management to smokers with strong quitting intentions. After the quit date, we sent messages on consciousness raising, dramatic relief, environmental re-evaluation and self-re-evaluation to smokers who had relapsed.All the participants also received messages by PMT related to the severity and susceptibility of smoking, intrinsic and extrinsic rewards of smoking, self-efficacy and response efficacy and response cost of quitting. The motivational messages consisted of 14 subgroups with a total of 400 text messages. A more detailed framework of the text and intervention based on the TTM and PMT is reported separately [