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E-cigarettes: an evidence update <br /> <br />19 <br />have been conducted approximately annually since that time. Probability sampling <br />methods are utilised through telephone surveys using random digit dialling, but in more <br />recent survey waves participants could opt to complete surveys on the internet. The ITC <br />UK study benefits from parallel cohort surveys in Australia, Canada and the United <br />States, enabling comparisons across countries with different tobacco and EC policies. <br />Each wave of the survey includes approximately 1,500 UK respondents. EC questions <br />were added to the last three waves. Data from the last wave (in 2014) were not <br />available for inclusion in this report, but published papers from earlier waves are <br />included. More details of the methodology are available at www.itcproject.org <br /> <br /> <br /> <br /> <br />