title: Web-based self-reported height, weight, and body mass index among Swedish adolescents: a validation study.
abstract: BACKGROUND
  Web-collected height and weight are increasingly used in epidemiological studies; however, the validity has rarely been evaluated.
  
  
  OBJECTIVE
  The aim of the study was to validate self-reported height, weight, and corresponding body mass index (BMI) among Swedish adolescents aged approximately 16 years. A secondary aim was to investigate possible prediction factors for validity of self-reported BMI.
  
  
  METHODS
  The study included 1698 adolescents from the population-based cohort BAMSE. Height and weight were collected through a Web-based questionnaire and subsequently measured using standard procedures. Differences between reported and measured height, weight, and corresponding BMI were compared by t tests and agreement was evaluated by Pearson correlation and Bland-Altman plots. Multivariable linear regression analysis was used to investigate whether lifestyle and demographic factors predicted validity of self-reported BMI.
  
  
  RESULTS
  On average, weight was underestimated by 1.1 kg and height was overestimated by 0.5 cm, leading to an underestimation of BMI by 0.5 kg/m2. Correlation coefficients were .98 for height, .97 for weight, and .94 for BMI, and highly significant. Females underestimated weight to a higher extent than males and overweight and obese participants underestimated weight to a higher extent than normal-weight participants, which resulted in higher underestimation of BMI. Underweight participants, on the contrary, overestimated weight and correspondingly BMI. Overall, a high proportion of participants were classified into the correct BMI category; however, among overweight and obese participants, only 60.2% (139/231) and 46% (20/44) were correctly classified, respectively. In the multivariable prediction model, only gender and BMI status significantly predicted discrepancy between reported and measured BMI.
  
  
  CONCLUSIONS
  Web-collected BMI may be used as a valid, quick, and cost-effective alternative to measured BMI among Swedish adolescents. The accuracy of self-reported BMI declines with increasing BMI and self-reported BMI should not be used to estimate the prevalence of overweight or obesity.
authors: Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden. Sandra.Ekstrom@ki.se.
authors: Sandra
authors: http://orcid.org/0000-0002-2060-8190
authors: S
authors: Ekström
authors: Inger
authors: http://orcid.org/0000-0001-6096-3771
authors: I
authors: Kull
authors: Sara
authors: http://orcid.org/0000-0002-0695-4134
authors: S
authors: Nilsson
authors: Anna
authors: http://orcid.org/0000-0002-7981-6314
authors: A
authors: Bergström
mesh_terms: D000293:Adolescent; D000328:Adult; D001827:Body Height; D015992:Body Mass Index; D001835:Body Weight; D005260:Female; D006801:Humans; D020407:Internet; D008297:Male; D015203:Reproducibility of Results; D057566:Self Report; D011795:Surveys and Questionnaires; D013548:Sweden; D055815:Young Adult
keywords: Internet; adolescent; body height; body mass index; body weight; validity