Design, setting and participants

The cross-sectional study was performed in May 2017 in a sample of 4058 respondents aged ≥18 years. The population of the survey was inhabitants in the Attica prefecture. This region hosts 35% of the total adult population of Greece according to Census 2011 (http://www.statistics.gr/en/2011-census-pop-hous) and consists of the Regional Departments of Athens, Piraeus, Eastern Attica and Western Attica. The sample drawn was representative of the prefecture of Attica population, with all registered landline phone numbers being used as the sampling frame. According to the Hellenic Authority for Telecommunications and Postal services (www.eett.gr) landline phones penetration in 2016 was 44.1% of the population, while mobile phones was 148%. The above difference is a common example for a possible coverage error [22]. For the landline phones, the potential issue on the survey is the under-coverage, but for the usage of mobiles phones, it is the over-coverage. Unfortunately, there is no reliable benchmark study for the Greek population that identifies the direction, strength and potential implications of the coverage error. Using the landline phones prevented the error of over-presenting into the sample people that own more than one mobile phone. The mode of data collection was computer-assisted telephone interviews (CATI), and the sample design was stratified random sampling. Further post-survey adjustment weights were applied in order to depict sample composition as the actual population demographics.

The study was approved by the ethics committee of the Onassis Cardiac Surgery Center (reference no. 591/14.12.16). All participants provided verbal consent at the beginning of the interview in order to participate to this study.

Sociodemographic factors

The exact age of the participants was recorded and then recoded into five categories (18–24, 25–39, 40–54 and 55 years and older). Education was coded as “high school or less”, “technical education”, “university education” and “postgraduate education”. Marital status was recorded and categorized as “single”, “married/living with a partner” and “widowed/divorced”. The financial status of the participants was assessed through a question seeking a self-assessment report of the financial conditions of each household: “How would you characterize your financial situation?”. Response options were “We are unable to cope with our household finances” (very bad financial situation), “We are able to cope with our household finances, but with a lot of difficulties” (bad financial situation), “We are able to to cope with our household finances, but without the ability to save a lot of money” (not good financial situation), and “We don’t have financial problems” (good financial situation). This variable attempts to accommodate measurement issues, processing errors and item non-response errors [22, 23]. For the analysis, responses were recoded into three groups (“very bad/bad”, “not good”, and “good”).

Smoking and e-cigarette use

Participants were categorized according to their smoking status as current smokers, past smokers and never smokers. Current and former smokers were asked whether they were smoking daily or occasionally. Current daily smokers were asked whether they had tried to quit smoking in the past 3 years and, if yes, which smoking cessation methods they had used. Response options were “Nicotine replacement therapies”, “Oral smoking cessation medications”, “Psychological support”, “E-cigarettes” and “By myself (without any aid)”, with participants being able to choose multiple responses.

E-cigarette awareness was assessed by asking: “Have you ever heard of e-cigarettes?”. Those responding “No” were classified as never e-cigarette users and no further questions about e-cigarettes were asked. E-cigarette use was assessed by asking: “Regarding the use of e-cigarettes, which of the following statements applies to you?” The responses “I currently use e-cigarettes” (current use), “I used them in the past, but no longer use them” (past use) and “I tried them in the past only once or twice” (past experimentation) were categorized as ever e-cigarette use. The response “I have never used them” was categorized as never use. A separate question assessed the use of nicotine-containing e-cigarettes among ever e-cigarette users, by asking “Do/did you use nicotine in your e-cigarette?” Current and past e-cigarette users were asked whether they use or used the e-cigarette daily or occasionally. The harm perception for e-cigarettes was examined by asking “Do you think e-cigarettes are…”, with available responses being “A lot more harmful than tobacco cigarettes”, “More harmful than tobacco cigarettes”, “As harmful as tobacco cigarettes”, “Less harmful than tobacco cigarettes”, “A lot less harmful than tobacco cigarettes”, “Absolutely harmless” and “Don’t know”.

Statistical analysis

All values were presented as weighted proportions with 95%CI, while the projected number within the Attica population was extrapolated based on the total sampling population using an appropriate weighing variable. Differences in prevalence of e-cigarette current use, past use and past experimentation were assessed using chi-square (χ2) test. Two separate logistic regression analyses were performed to assess correlates of ever and current e-cigarette use, similarly to a previous analysis of data from Eurobarometer 2014 [24]. Independent variables included all demographics as well as the perception of e-cigarette harmfulness. The latter was recoded as a binary variable, with responses “A lot more harmful than tobacco cigarettes”, “More harmful than tobacco cigarettes”, “As harmful as tobacco cigarettes” and “Don’t know” coded as 1 and “Less harmful than tobacco cigarettes”, “A lot less harmful than tobacco cigarettes” and “Absolutely harmless” coded as 2. All analyses were weighted and were performed with commercially available software (SPSS v.22.0, Chicago, IL, USA).