Procedure
SHOP
Participants were recruited and screened for eligibility by a research agency (The study was presented as investigating how cigarette pack size affects the effectiveness of health warnings. This was to reduce the chance of participants focusing on their cigarette consumption in relation to pack size. Potential participants provided information on their demographic characteristics and smoking behaviour. Potential participants were asked to purchase one pack of 20 and one pack of 25 cigarettes in their usual brand variant. They sent photographs of these packs and their receipts to the research team. This was to check the eligibility criterion that they used a brand or brand variant available to buy in both pack sizes from a shop convenient to them.