Randomization
Allocation of participants to the order in which they completed the conditions was determined using a computer‐generated random number sequence prepared by one of the project statisticians (R.M.), using Stata version 15 (StataCorp LLC, TX, USA). Block randomization was used to generate an equal number of participants allocated to each treatment order and to reduce the potential of selection bias compared to simple randomization. The blocks were in sizes of 2, 4 and 6. The random number sequence, with IDs for the sequence of potential participants, was concealed from the research team, research agency and participants until the participant had consented to take part in the study and shown that they were able to purchase their usual brand variant of cigarettes in packs of 20 and packs of 25. When a participant was deemed eligible for randomization the research team accessed the next random allocation in the sequence, and this participant was assigned the corresponding ID. Instructions (see Supporting information, Material The stickers had space to record the following information: (i) participant ID; (ii) pack number; (iii) date the pack was finished; (iv) a rating from 1 to 7 for the effectiveness of the warning label (to bolster the credibility of the cover story); (v) number of cigarettes smoked themselves from the pack; (vi) number of cigarettes smoked from other non‐study packs while this study pack was open (e.g. given to them by a friend); and (vii) number of cigarettes remaining (for partially empty packs at the end of a study period).On day 1 of each intervention period, participants were e‐mailed a reminder to only smoke their usual cigarette brand variant of cigarettes from packs of the appropriate size, as randomized.On day 7 of each intervention period, participants sent photographs of all the cigarette packs they had finished since day 1. After smoking the last cigarette on day 14, participants sent photographs of all the cigarette packs they had smoked from since day 7, including any partially empty packs.At the end of each intervention period, participants completed questionnaires about their smoking and cigarette purchasing during the study. Participants received Can$400 for completing the study procedures, and purchased their own cigarettes during the study.