The Cardiff Study of All Wales and North-west England Twins (CaStANET) is a population-based register of twin births between 1980 and 1991 in Wales and Greater Manchester, UK [31]. Information was used from the fourth wave (2004) of data collection on the substance use of the twins. A total of 1755 families with twins aged 11–19 years were contacted with mailed questionnaires. Non-responders were initially sent reminders, then remailed the questionnaire. Families received £15 ‘thank you’ payments in high street vouchers as a token of appreciation.
