Marques-Vidal et al. BMC Public Health 2011, 11:772 http://www.biomedcentral.com/1471-2458/11/772 Page 4 of 8 Table 1 Characteristics of the participants and prevalence of overweight and obesity, overall and by gender Sample size Women (%) Age group (%) 18-24 25-34 35-44 45-54 55-64 65-74 75+ Educational level (%) Primary Secondary University BMI (kg/m2) Average ± SD Median (IQR) BMI categories (%) Normal Overweight Obese Men BMI (kg/m2) Average ± SD Median (IQR) BMI categories (%) Normal Overweight Obese Women BMI (kg/m2) Average ± SD Median (IQR) BMI categories (%) Normal Overweight Obese 1995-6 38,504 20,451 (53.1) 4,962 (12.9) 5,389 (14.0) 6,319 (16.4) 6,319 (16.4) 6,445 (16.7) 5,567 (14.5) 3,503 (9.1) 28,570 (74.2) 7,293 (18.9) 2,641 (6.9) 25.2 ± 4.0 24.8 (22.5 - 27.5) 20,164 (52.4) 13,909 (36.1) 4,431 (11.5) 25.4 ± 3.5 25.1 (23.1 - 27.5) 8,887 (49.2) 7,310 (40.5) 1,856 (10.3) 25.0 ± 4.4 24.4 (21.9 - 27.5) 11,277 (55.1) 6,599 (32.3) 2,575 (12.6) 1998-9 38,688 20,556 (53.1) 4,685 (12.1) 5,722 (14.8) 6,344 (16.4) 6,379 (16.5) 6,122 (15.8) 5,707 (14.8) 3,729 (9.6) 26,682 (69.0) 8,787 (22.7) 3,219 (8.3) 25.4 ± 4.1 25.0 (22.6 - 27.7) 19,420 (50.2) 14,312 (37.0) 4,956 (12.8) 25.7 ± 3.7 25.4 (23.2 - 27.7) 8,358 (46.1) 7,702 (42.5) 2,072 (11.4) 25.2 ± 4.5 24.7 (22.0 - 27.9) 11,062 (53.8) 6,610 (32.2) 2,884 (14.0) 2005-6 25,348 13,322 (52.6) 3,222 (12.7) 3,309 (13.1) 3,934 (15.5) 4,249 (16.8) 3,906 (15.4) 3,917 (15.5) 2,811 (11.1) 15,060 (59.4) 7,472 (29.5) 2,816 (11.1) 25.7 ± 4.5 25.2 (22.6 - 28.1) 12,294 (48.5) 9,233 (36.4) 3,821 (15.1) 25.9 ± 4.1 25.6 (23.3 - 28.3) 5,330 (44.3) 5,006 (41.6) 1,690 (14.1) 25.4 ± 4.8 24.8 (22.0 - 28.1) 6,964 (52.3) 4,227 (31.7) 2,131 (16.0) Test (p-value) 2.47 (0.30) 146.3 (<0.001) 1557.3 (<0.001) 87.9 (<0.001) 202.4 (<0.001) 61.8 (<0.001) 138.4 (<0.001) 32.8 (<0.001) 80.18 (<0.001) Obesity was defined as a body mass index ≥30 kg/m2. Results are expressed as number of participants and (percentage) or as mean ± standard deviation. IQR: interquartile range. Statistical analysis by chi-square or analysis of variance. reported BMI underestimates the true prevalence of obesity in a population, still it can be used to assess trends [23] and to compare between countries [4], pro- vided the same methodology of data collection is used between studies, which was here the case. Finally, although comparison with international data might be difficult, our data indicate that, in 2005-6, the overall prevalence of reported obesity in Portugal was higher than in Switzerland but comparable to France (Table 6). Contrary to obesity, overweight rates tended to stabi- lize or even to decrease slightly in the overall population. These findings do not replicate the ones from a previous study, which showed an increase in the prevalence of overweight in the Portuguese population from 35.2% in 1995-8 to 39.4% in 2003-5 [2], but the precise rationale for this stabilization is currently unknown. A possible explanation would be that the magnitude of the self-reporting bias for height and weight increased between 1995-6 and 2005-6 as noted in other studies [26], but we lack data to verify this hypothesis. This increase in self-reporting bias would have lessened the observed increase in BMI and obesity