National Health and Nutrition Examination Survey, 2015−2018 : sample design and estimation procedures
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National Health and Nutrition Examination Survey, 2015−2018 : sample design and estimation procedures

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    Background: The purpose of the National Health and Nutrition Examination Survey (NHANES) is to produce national estimates representative of the total noninstitutionalized civilian U.S. population. The sample for NHANES is selected using a complex, four-stage sample design. NHANES sample weights are used by analysts to produce estimates of the health-related statistics that would have been obtained if the entire sampling frame (i.e., the noninstitutionalized civilian U.S. population) had been surveyed. Sampling errors should be calculated for all survey estimates to aid in determining their statistical reliability. For complex sample surveys, exact mathematical formulas for variance estimates that fully incorporate the sample design are usually not available. Variance approximation procedures are required to provide reasonable, approximately unbiased, and design- consistent estimates of variance. Objective: This report describes the NHANES 2015−2018 sample design and the methods used to create sample weights and variance units for the public-use data files, including sample weights for selected subsamples, such as the fasting subsample. The impacts of sample design changes on estimation for NHANES 2015−2018 are described. Approaches that data users can use to modify sample weights when combining survey cycles or when combining subsamples are also included. Methods: Log-binomial regression models were used to examine the impact of the staggered implementation of the pregnancy checkbox by states over time and to predict MMRs under two alternative scenarios: (1) assuming that no states had the checkbox at any point, and (2) assuming that all states had the checkbox from 1999 through 2017. The impact of the checkbox and related trends over time were examined by age, race and Hispanic origin, state of occurrence, and causes of maternal death. Sensitivity analyses examined the impact of outcome misclassification. Results: The implementation of the checkbox was associated with an increased identification of maternal deaths. Averaging over the period 2003–2017, the checkbox resulted in an MMR increase of 9.6 deaths per 100,000 live births (95% confidence interval: 8.6–10.6). The average impact of the checkbox adoption was greater for women aged 40 and over, non-Hispanic black women, and for certain causes of death. Accounting for the checkbox, predicted MMRs did not change significantly from 1999 through 2017, although trends varied by subgroup (age, race and Hispanic origin, cause of death). Conclusions: Estimated trends suggest that the observed increases in MMRs from 1999 through 2017 reported in the literature were largely due to the staggered implementation of the checkbox. Potential misclassification of pregnancy status using the pregnancy checkbox likely also contributed, which disproportionately inflated MMRs among women aged 40 and over. Suggested citation: Chen TC, Clark J, Riddles MK, Mohadjer LK, Fakhouri THI. National Health and Nutrition Examination Survey, 2015−2018: Sample design and estimation procedures. National Center for Health Statistics. Vital Health Stat 2(184). 2020. CS315792 sr02-184-508.pdf
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