title: Comparative effects of the restriction method in two large observational studies of body mass index and mortality among adults.
abstract: A method applied in some large studies of weight and mortality is to begin with a well-defined analytic cohort and use successive restrictions to control for methodologic bias and arrive at final analytic results.
  
  
  
  Two observational studies of body mass index and mortality allow a comparative assessment of these restrictions in very large data sets. One was a meta-analysis of individual participant data with a sample size of 8 million. The second was a study of a South Korean cohort with a sample size of 12 million. Both presented results for participants without pre-existing disease before and after restricting the sample to never-smokers and deleting the first 5 years of follow-up.
  
  
  
  Initial results from both studies were generally similar, with hazard ratios (HRs) below 1 for overweight and above 1 for underweight and obesity. The meta-analysis showed higher HRs for overweight and obesity after the restrictions, including a change in the direction of the HR for overweight from 0·99 (95% CI: 0·98-1·01) to 1·11 (95% CI: 1·10, 1·11). The South Korean data showed little effect of the restrictions and the HR for overweight changed from 0·85 (95% CI: 0·84-0·86) to 0·91 (95% CI: 0·90, 0·91). The summary effect size for overweight was 0·90 (95% CI: 0·89-0·91) before restrictions and 1·02 (95% CI: 1·02, 1·03) after restrictions.
  
  
  
  The effect of the restrictions is not consistent across studies, weakening the argument that analyses without such restrictions lack validity.
authors.affiliation: Stanford Prevention Research Center, Department of Medicine, Stanford University School of Medicine, Stanford, CA, USA.
authors.forename: Katherine M
authors.identifier: 0000-0002-0838-469X
authors.initials: KM
authors.lastname: Flegal
authors.affiliation: Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, MD, USA.
authors.forename: Barry I
authors.initials: BI
authors.lastname: Graubard
authors.affiliation: Department of Preventive Medicine and Public Health, Catholic Kwandong University College of Medicine, Gangneung, Korea.
authors.forename: Sang-Wook
authors.initials: SW
authors.lastname: Yi
mesh_terms: D000328:Adult; D015992:Body Mass Index; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D015201:Meta-Analysis as Topic; D009765:Obesity / Q000401:mortality; D064887:Observational Studies as Topic; D050177:Overweight / Q000401:mortality*; D013851:Thinness / Q000401:mortality*
keywords: Body mass index; comparative analysis; epidemiologic methods; mortality