NHANES body measurement data are used to monitor trends in infant and child growth, to estimate the prevalence of overweight and obesity in the U.S. population, and to examine the associations between body weight and the health and nutritional status of the U.S. population. The target age groups for the NHANES 2007-2008 body measurement component are as follows:
Head circumference: birth through 6 months of age
Mid-upper arm circumference: 2 months of age and older
Recumbent length: birth through 47 months
Standing height: 2 years and older
Subscapular skin fold: 2 months of age and older
Triceps skin fold: 2 months of age and older
Upper arm length: 2 months of age and older
Upper leg length: 8 years and older
Waist circumference: 2 years of age and older
Weight: All ages
All survey participants were eligible for the body measurement 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. Maximal calf and mid-thigh circumference measurements were discontinued after the NHANES 2005-2006 data collection cycle was completed.
Protocol and Procedure
The body measurement data were collected by trained health technicians. The health technician was assisted by a recorder during the body measurement examination. The respondent’s age at the time of the screening interview determined the body measurement examination protocol for survey participants. In some instances the age at screening interview and age at the time of the health examination differed by several weeks. The Demographics data file includes variables for age at screening (RIDAGEYR and RIDAGEMN) and age at examination (RIDAGEEX).
Arm and leg measurements were made on the right side of the body. If an examinee 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 examiner took measurements on the left side. The body measurements file does not identify persons who had amputations because the information might be considered identifiable and pose a disclosure concern. The body weight data for individuals who had limb amputations were set to missing.
This data file includes body measurements 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 Demographic file variable, RIDEXPRG - pregnancy status at the time of the health examination. 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 individuals are not reported.
For general guidelines on the body measurement procedures, please refer to the Anthropometric Standardization Reference Manual (Lohman et al, 1988).
The NHANES III Anthropometric Procedures Video (Centers for Disease Control and Prevention) illustrates the NHANES III anthropometry methodology. The same methods have been used in each of the continuous NHANES cycles. The NHANES III anthropometry component included several measurements that are no longer part of the anthropometry protocol. The NHANES III video is available in VHS and DVD format upon request and may also be viewed on the NHANES website.
The NHANES 2007-2008 Body Measurement Procedures Manual is posted on the NHANES 2007-2008 website. The manual provides descriptions of the NHANES protocol, survey equipment, and quality control procedures.
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, using the NHANES III anthropometry video, 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 mobile examination centers, 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 measurement examination rooms in each of the mobile examination centers were identical with respect to layout and equipment. Scheduled equipment calibration was performed by the health technicians and verified by supervisory staff. The Body Measures Procedures Manual includes detailed descriptions of the quality assurance and quality control measures that are used in NHANES.
Data Processing and Editing
The 2007-2008 data were reviewed for unusual and erroneous values. Review criteria, based on NHANES III and NHANES 1999–2006 body measures data, were used for the initial data reviews. Range checks for infants under 2 months of were based on NHANES 1999–2006 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.
Component status code: A final body measures component status code (BMDSTAT) provides analysts with a quick method of identifying survey participants with complete or partial body measurement data.
Unusual values: Unusual body measurement values were noted during the review of the data. Typically, unusual values occurred when a subject was extremely short, tall, overweight or underweight. Analysts should examine the distributions of the body measurements carefully. In particular, the upper arm length (BMXARML) and upper leg length (BMXLEG) values are affected by extreme amounts of adipose tissue. Skinfold measurements were not obtained for many obese subjects because the amount of adipose tissue exceeded the limits of the caliper, or the technician was not able to grasp a double thickness of tissue.
Comment Codes: Comment codes were added by the health technicians, during data collection, to document problems or situations that arose during the body measurement examination. For example, the variable BMIWT is a comment code for the body weight measurement. If a subject 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 Analytic Guidelines for further details on the use of sample weights and other analytic issues. The Analytic Guidelines are available on the NHANES website.