Open Access Research article

Assessment of selection bias in a health survey of children and families – the IDEFICS Sweden-study

Susann Regber1*, Masuma Novak2, Gabriele Eiben3, Lauren Lissner3, Sabrina Hense4, Tatiana Zverkova Sandström1, Wolfgang Ahrens4 and Staffan Mårild5

Author Affiliations

1 Nordic School of Public Health NHV, Box 12 133, SE- 402 42, Gothenburg, Sweden

2 Department of Molecular and Clinical Medicine, the Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

3 Department of Public Health and Community Medicine, Public Health Epidemiology Unit Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

4 Department of Epidemiological Methods and Etiologic Research, Institute for Epidemiology and Prevention Research (BIPS), Bremen, Germany

5 Department of Paediatrics, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden

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BMC Public Health 2013, 13:418  doi:10.1186/1471-2458-13-418

Published: 1 May 2013



A health survey was performed in 2007–2008 in the IDEFICS/Sweden study (Identification and prevention of dietary- and lifestyle-induced health effects in children and infants) in children aged 2–9 years. We hypothesized that families with disadvantageous socioeconomic and -demographic backgrounds and children with overweight and obesity were underrepresented.


In a cross-sectional study, we compared Swedish IDEFICS participants (N=1,825) with referent children (N=1,825) using data from Statistics Sweden population registers. IDEFICS participants were matched for age and gender with a referent child living in the same municipality. Longitudinal weight and height data from birth to 8 years was collected for both populations (n=3,650) from the children’s local health services. Outcome measures included the family’s socioeconomic and demographic characteristics, maternal body mass index (BMI) and smoking habits before pregnancy, the children’s BMI standard deviation score (SDS) at the age of inclusion in the IDEFICS study (BMISDS-index), and the children’s BMI-categories during the age-span. Comparisons between groups were done and a multiple logistic regression analysis for the study of determinants of participation in the IDEFICS study was performed.


Compared with IDEFICS participants, referent families were more likely to have lower education and income, foreign backgrounds, be single parents, and have mothers who smoked before pregnancy. Maternal BMI before pregnancy and child’s BMISDS-index did not differ between groups. Comparing the longitudinal data-set, the prevalence of obesity was significantly different at age 8 years n= 45 (4.5%) versus n= 31 (2.9%) in the referent and IDEFICS populations, respectively. In the multivariable adjusted model, the strongest significant association with IDEFICS study participation was parental Swedish background (odds ratio (OR) = 1.91, 95% confidence interval (CI) (1.48–2.47) followed by parents having high education OR 1.80, 95% CI (1.02-3.16) and being married or co-habiting OR 1.75 95% CI (1.38-2.23).


Families with single parenthood, foreign background, low education and income were underrepresented in the IDEFICS Sweden study. BMI at inclusion had no selection effect, but developing obesity was significantly greater among referents.

Selection bias; Children; Obesity; Health survey; Registers