title: Impact of instrument error on the estimated prevalence of overweight and obesity in population-based surveys.
abstract: BACKGROUND
  The basis for this study is the fact that instrument error increases the variance of the distribution of body mass index (BMI). Combined with a defined cut-off value this may impact upon the estimated proportion of overweight and obesity. It is important to ensure high quality surveillance data in order to follow trends of estimated prevalence of overweight and obesity. The purpose of the study was to assess the impact of instrument error, due to uncalibrated scales and stadiometers, on prevalence estimates of overweight and obesity.
  
  
  METHODS
  Anthropometric measurements from a nationally representative sample were used; the Norwegian Child Growth study (NCG) of 3474 children. Each of the 127 participating schools received a reference weight and a reference length to determine the correction value. Correction value corresponds to instrument error and is the difference between the true value and the measured, uncorrected weight and height at local scales and stadiometers. Simulations were used to determine the expected implications of instrument errors. To systematically investigate this, the coefficient of variation (CV) of instrument error was used in the simulations and was increased successively.
  
  
  RESULTS
  Simulations showed that the estimated prevalence of overweight and obesity increased systematically with the size of instrument error when the mean instrument error was zero. The estimated prevalence was 16.4% with no instrument error and was, on average, overestimated by 0.5 percentage points based on observed variance of instrument error from the NCG-study. Further, the estimated prevalence was 16.7% with 1% CV of instrument error, and increased to 17.8%, 19.5% and 21.6% with 2%, 3% and 4% CV of instrument error, respectively.
  
  
  CONCLUSIONS
  Failure to calibrate measuring instruments is likely to lead to overestimation of the prevalence of overweight and obesity in population-based surveys.
authors: Division of Epidemiology, Norwegian Institute of Public Health, Nydalen, Oslo, 0403, Norway. anbi@fhi.no
authors: Anna
authors: A
authors: Biehl
authors: Ragnhild
authors: R
authors: Hovengen
authors: Haakon E
authors: HE
authors: Meyer
authors: Jøran
authors: J
authors: Hjelmesaeth
authors: Jørgen
authors: J
authors: Meisfjord
authors: Else-Karin
authors: EK
authors: Grøholt
authors: Mathieu
authors: M
authors: Roelants
authors: Bjørn Heine
authors: BH
authors: Strand
mesh_terms: D000886:Anthropometry / Q000295:instrumentation*; D015992:Body Mass Index; D002138:Calibration / Q000592:standards; D002648:Child; D003198:Computer Simulation; D005260:Female; D006801:Humans; D008297:Male; D009664:Norway / Q000453:epidemiology; D009765:Obesity / Q000453:epidemiology*; D050177:Overweight / Q000453:epidemiology*; D011159:Population Surveillance / Q000379:methods*; D015995:Prevalence; D015203:Reproducibility of Results