The data collection protocol included two phases, a telephone screening and a diagnostic interview phase, after which a sample of blood for DNA extraction was obtained (via phlebotomy lab or public health clinic selected by the respondent). Eligible families across both sites were required to have at least one adult sibpair (including no more than one monozygotic twin per pair) concordant for a history of cigarette smoking, based on earlier questionnaire or interview surveys. At the Australian site, families were targeted for screening using the criteria that they have at least one sibpair possibly concordant for a lifetime history of heavy smoking (defined as either a history of smoking 20 cigarettes per day during the period of heaviest smoking, or smoking at least 40 cigarettes in any 24-hour period). Priority was given to families with available biological parents. On the Finnish site, due to the absence of information on cigarette smoking history in non-twin siblings (from previous surveys), families with dizygotic co-twins concordant for a lifetime history of regular smoking were prioritized. Some families with monozygotic co-twins concordant for lifetime
