Evolution of socioeconomic inequalities in smoking: results from the Portuguese national health interview surveys

We used data from the four National Health Interview Surveys (NHIS) carried out so far in all mainland Portuguese regions (1987, 1995, 1998/99, and 2005/06). NHIS are cross-sectional studies based on representative samples of non-institutionalized population living in Portugal. Data were collected through face-to-face interviews on health status and disease, socio-demographic indicators and lifestyle, among others. The data from the NHIS are collected by the National Institute for Statistics, available on demand for research purposes, and the methods are reported elsewhere [16]. In this study, all individuals aged 25 to 79 years old were included; younger people were excluded to avoid as much as possible individuals who had not completed their education. Also, older people were excluded to reduce the selective mortality bias. The final sample included 120,140 individuals.

We created three dichotomic variables for smoking: “current”, “former” and “ever” smoker. The “current smoker” variable was based on the question “do you currently smoke?” The last three surveys distinguished the answers “daily”, “occasionally” and “non-smoker”. However, the 1987 NHIS only considered two categories, “daily” and “non-smoker”. In order to compare the four samples, the “current smoker” variable has a value one when the person answers “daily smoker” to the above-referred question, and zero otherwise. The “daily smoker” category included a large majority of smokers (89.4% of smokers smoked daily in 1995, 89.1% in 1998–1999 and 90.3% in 2005–2006), so that the loss of information was relatively minor. In regard to the “former smoker” variable, we used the question “Did you ever smoke?”. This question was asked solely to those who did not currently smoke; possible answers were “daily”, “occasionally”, or “never smoked”. The 1987 NHIS asked “Did you ever smoked regularly?”; hence, the “former smoker” variable has a value one for those who answered “yes” in 1987 and those who answered “daily” in subsequent surveys, and zero otherwise. Occasional smokers were included as never smokers. Finally, the “ever smoker” variable values one if the persons reports to be current or former smoker, and zero otherwise. Current smoking is thus given by the percentage of daily smokers in the sample, ever smoking by the percentage of ever smokers (current or former) within the respondents, and smoking cessation by the percentage of former smokers within the ever smokers.

We categorized education into five categories, on the basis of the highest completed diploma, namely no education (zero to three years of education), pre-primary education (four years of education), primary education (nine years of education), secondary education (12 years of education), and tertiary education (more than 12 years of education). The individual income was calculated applying the Organisation for Economic Co-operation and Development (OECD) modified equivalence scale, giving different weights to different family members (1 to the first adult, 0.5 to the second and other family members older than 14 years old, and 0.3 to individuals less than 14 years old) [17]. In all surveys, the upper income category was open ended, e.g., “€2,000 and above”. To estimate the midpoint for the upper income category we followed Parker and Fenwick method [18].

Socioeconomic inequalities in current smoking and smoking cessation were measured using relative inequality indexes (RII), concentration indexes (CI), and odds ratios (OR). Logistic regressions were performed in order to estimate age-adjusted OR of smoking. The dependent variables were current smoking, smoking cessation and ever smoking. The independent variables were age and education, or age and income. Separate analyses were performed for men and women, for each survey, and for different birth cohorts.

For the sake of brevity, we only display results for RII and CI. An additional file shows results for OR analyses [see Additional file 1].

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