Generalisability and limitations
BLIND, ATTRITION
Participants were recruited online and through primary health care centres, mimicking closely how the intervention could be disseminated in the real-world. Inclusion criteria were allowing, and few participants who showed interest in the trial were excluded. This pragmatic trial design leads us to interpret findings as estimates of effectiveness rather than efficacy, which strengthens the external validity of the trial. The design did however also limit our ability to blind participants, as they were naturally aware if they received the intervention or not. Although one could imagine some form of sham intervention with text messages containing non-smoking related information, it is unlikely that this would convince those seeking help that they had received cessation support.Attrition rates were high in this trial, as is often the case with online trials with low thresholds of participation [Outcome measures were self-reported in this trial, which may be susceptible to recall bias and exacerbate the risk of social desirability bias. The Society for Research on Nicotine and Tobacco does however recommend that, in studies with limited face-to-face contact, it is neither required nor desirable to use biochemical verification [All study procedures were automated, ensuring that the concealment of the allocation sequence could not be subverted nor that research personnel could become aware of allocation. However, to reduce attrition bias, we called participants who did not respond to our initial attempts to collect data. This induces a risk of detection bias, which has been shown to affect outcome reporting [