Marques-Vidal et al. BMC Public Health 2011, 11:772 http://www.biomedcentral.com/1471-2458/11/772 been published for the overall Portuguese population. As a lower educational level is associated with worse health behaviours [12-14] and to a lower awareness of the health impact of excess weight [15,16] it is important to assess the impact of education on obesity levels and trends. In this study, we assessed the 10-year trends in the prevalence of overweight and obesity in the Portuguese population, overall and by educational level, using data from the Portuguese National Health Surveys. The information collected will allow a better allocation of resources aimed at preventing the obesity epidemic in Portugal, by targeting specific groups and/or tailoring the preventive messages. Methods National Interview Surveys The Portuguese National Health Survey is an official survey conducted by the Portuguese National Institute of Statistics in collaboration with the National Health Observatory under the responsibility of the Portuguese Ministry of Health. The National Health Survey provides information for the planning of the Portuguese health system and also to the WHO, Eurostat and OECD. The survey is anonymous and as part of the official statistics no approval from an ethics committee is necessary. The methodology of the Portuguese national interview surveys has been described previously [9,17]. Briefly, the National Health Surveys were conducted between May 1995 and April 1996 (1995-6), October 1998 and Sep- tember 1999 (1998-9) and February 2005 and January 2006 (2005-6). The sampling frame was built on census data and included all subjects living in individual hous- ing during that period (collective housing such as hospi- tals, prisons, military barracks, or retirement houses were excluded). The sample was considered representa- tive of the main regions of continental Portugal (North, Center, Lisbon region, Alentejo, and Algarve). In the 2005-6 survey the autonomous regions of the Azores and Madeira were also included, but not considered in the present analysis. The primary sampling unit (PSU) was the house, and data were derived from the popula- tion and housing census. Within each main region, two strata were defined: the freguesias (corresponding to counties) and, within the freguesias, geographically defined units of ± 300 lodgings (240 in 2005-6). The PSUs were then randomly selected within each geogra- phically defined unit. All subjects living in the sampling unit (house) were surveyed. All surveys were carried out in compliance with the Helsinki Declaration. Data were collected by trained staff according to a standardized protocol [17], and quality control was maintained by reapplying (by a different interviewer) the same ques- tionnaire to 10% of the initial sample. Participation rates Page 2 of 8 (defined as the percentage of households who responded) as reported by the National Institute of Sta- tistics were 88% in 1995-6, 82% in 1998-9 and 76% in 2005-6. In the 2005-6 survey, participation rates were 83% for North, 76% for center, 70% for Lisbon and Tagus valley and 78% for Alentejo and Algarve; no data was available for the previous surveys. Sample size was proportional to the population of each region in the 95- 96 and 98-99 surveys, which was not exactly the case in the 2005-6 survey. As sampling weights were only avail- able for the 2005-6 survey [17], the original (unweighted) data was analyzed. Data were provided upon request by the National Health Observatory (http://www.onsa.pt) (1995-6 and 1998-9) and the National Institute of Statistics (http:// www.ine.pt) (2005-6). In 1995-6, 49,718 subjects were surveyed, of whom 38,504 (18,053 men and 20,451 women) were aged over 18 years and had reported height and weight measurements; the corresponding fig- ures for 1998-9 were 48,606 and 38,688 (18,132 men and 20,556 women), respectively, and for 2005-6 they were 29,908 and 25,348 (12,026 men and 13,322 women). Data Collection All data were obtained by interview. Height and weight were self-reported; the correlations between self- reported and measured height, weight and BMI values are usually high, with correlation coefficients >0.9 [18,19]. Overweight was defined as a BMI ≥25 and <30 kg/m2; obesity was defined as a BMI ≥30 kg/m2 [20]. As self-reporting leads to an underestimation of obesity levels, several correction methods were used as in a pre- vious study [10]: 1) a second threshold of ≥29.2 kg/m2 to define obesity [21], 2) age and-gender specific correc- tions for height and weight as reported in a Portuguese study [22] 3) gender-specific corrections for height and weight [23] and 4) gender-specific BMI corrections of 0.8 kg/m2 and 1.1 kg/m2 in men and women, respec- tively [23]. The proposed corrections are within the values reported in a review of the literature [24]. Although self-reported BMI underestimates the true prevalence of obesity in a population, still it has been considered as valid to be used in epidemiological studies [25] to assess trends [23] and to compare between coun- tries [4]. Educational level was assessed by the number of years spent in school and classified into three groups: 6 years or less (primary school); 7 to 12 years (secondary school); and >12 years (university level). For the 1995-6 and 1998-9 surveys, age was provided as a continuous variable. For security reasons, it was not possible to obtain individual ages for the 2005-6 survey; hence, 10- year age groups were used. As the questionnaire on