NHANES body measures data are used to monitor trends in infant and child growth, to estimate the prevalence of overweight and obesity in U.S. children, adolescents, and adults, and to examine the associations between body weight and the health and nutritional status of the U.S. population.
The measurements and target age groups for the NHANES 2017–2018 body measures component are as follows:
- Weight: All ages
- Head circumference: birth through 6 months of age
- Recumbent length: birth through 47 months of age
- Standing height: 2 years and older
- Upper leg length: 8 years and older
- Upper arm length: 2 months of age and older
- Mid-upper arm circumference: 2 months of age and older
- Waist circumference: 2 years of age and older
- Hip circumference: 12 years of age and older
All survey participants were eligible for the body measures component. There were no medical, safety, or other exclusions for the body measurements protocol. The health technicians used their discretion to obtain as many measures as practical for persons who used a wheelchair.
Protocol and Procedure
The body measures data were collected, in the Mobile Examination Center (MEC), by trained health technicians. The health technician was assisted by a recorder during the body measures examination. The participant’s age at the time of the screening interview determined the body measures examination protocol. In some instances, the age at the screening interview and age at the time of the health examination differed by several weeks. The Demographics data file includes variables for age in years at screening (RIDAGEYR) for all participants and age in months at examination (RIDEXAGM) for participants who were less than 240 months of age at the time of examination.
Arm and leg measurements were made on the right side of the body. If a participant had an amputation, medical condition, or medical appliance, such as a cast, that prevented measurements from being taken on the right side of the body, the health technician took measurements on the left side. The body measurements file does not identify participants who had amputations because that information may be considered identifiable and pose a disclosure concern. The body weight data for participants who had limb amputations were set to “missing”.
This data file includes body measures for women who were pregnant at the time of their health examination. Pregnancy status at the time of the health examination is indicated by the variable, RIDEXPRG, in the Demographic data file. RIDEXPRG values are reported for women 20–44 years of age. RIDEXPRG for several pregnant women who were outside of this age range are not reported due to disclosure concerns. The body measures data for these participants are not reported.
Please refer to the NHANES 2017-2018 Anthropometry Procedures Manual for further details on obtaining body measurements.
Quality Assurance & Quality Control
The NHANES health technicians completed a 2-day training program with survey staff and an expert anthropometrist. The training included an overview of the component, general guidelines and technical skills for each measurement, and demonstrations conducted by the expert examiner with volunteer subjects. The expert examiner reviewed and demonstrated the proper technique to use for each measurement. Supervised practice exercises followed, conducted with several volunteer subjects, including infants, children, and adults. The chief health technician, at each of the MECs, monitored staff performance in the field. Health technician performance was also monitored using direct observation, data reviews, and periodic expert examiner (gold standard comparison) evaluations.
The body measures examination rooms in each of the MECs were identical with respect to layout and equipment. Scheduled equipment calibration was performed by the health technicians and verified by supervisory staff. The Anthropometry Procedures Manual includes detailed descriptions of the quality assurance and quality control measures that are used in the NHANES anthropometry/body measures component.
Data Processing and Editing
The 2017–2018 data were reviewed for unusual and erroneous values. Review criteria were based on the NHANES 1999–2016 body measurement data. During the data review, values that were above the 99th percentile or below the 1st percentile, for a particular age or age-gender group, were flagged for review. When records were flagged, the entire body measurements record was reviewed for reasonableness. Subject characteristics, such as height, weight, age, and gender were taken into consideration. Values that were determined to be unrealistic were deleted from the file. None of the original body measures data were changed and there are no imputed values in this file.
Body Mass Index (BMXBMI):
Body Mass Index (BMI) was calculated as weight in kilograms divided by height in meters squared, and then rounded to one decimal place.
Component status code: A final body measures component status code (BMDSTATS) provides analysts with a quick method of identifying survey participants with complete or partial body measurement data.
Unusual values: Unusual body measures values were noted during the review of the data. Typically, unusual values occurred when a subject was extremely short, tall, overweight, or underweight. In addition, the upper arm length (BMXARML) and upper leg length (BMXLEG) values may be affected by extreme amounts of adipose tissue. Analysts should examine the distributions of the body measurements carefully and consider whether or not it is appropriate to include or exclude extreme values in a given analysis.
Comment codes: Comment codes were added by the health technicians, during data collection, to document problems or situations that arose during the body measures examination. For example, the variable BMIWT is a comment code for the body weight measurement. If a participant did not change into the NHANES exam gown, a code of BMIWT of “3” denoting “clothing worn” was made in the record. Analysts should review the comment code information for each of the body measures prior to data analysis.
Sample weights: The NHANES examination sample weights should be used to analyze the body measurement data. Please refer to the NHANES Analytic Guidelines and the online NHANES Tutorial for further details on the use of sample weights and other analytic issues.