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Title: Comparative effects of the restriction method in two large observational studies of body mass index and mortality among adults.
Authors: Flegal KM,  Graubard BI,  Yi SW
Journal: Eur J Clin Invest
Date: 2017 Jun
Branches: BB
PubMed ID: 28380255
PMC ID: PMC5512593
Abstract: BACKGROUND: 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. MATERIALS AND METHODS: 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. RESULTS: 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 099 (95% CI: 098-101) to 111 (95% CI: 110, 111). The South Korean data showed little effect of the restrictions and the HR for overweight changed from 085 (95% CI: 084-086) to 091 (95% CI: 090, 091). The summary effect size for overweight was 090 (95% CI: 089-091) before restrictions and 102 (95% CI: 102, 103) after restrictions. CONCLUSIONS: The effect of the restrictions is not consistent across studies, weakening the argument that analyses without such restrictions lack validity.