MEC Operation and Schedule
NHANES examinations took place in the mobile examination center (MEC). The controlled environment of the MEC allowed physical measurements to be done under standardized conditions at each survey location. Eligibility for each examination component was determined by the participant's age at the time of screening and gender. The MEC was open 5 days per week; the non-operational days changed on a rotating basis so that appointments could be scheduled on any day of the week. Two examination sessions were conducted daily. Participants were randomly assigned to exams in the morning, or in the afternoon/evening session. Each examination session was 4 hours and usually included 10-12 sample participants. Participants did not receive components following a fixed order. Instead, at the beginning of each session, the MEC coordinator would assign participants to begin different components concurrently. Once a component was completed, the participant was assigned to another room based on the availability and exam-specific requirements. For example, a urine test confirming non-pregnant status needed to be completed before a female participant could be assigned to the liver elastography exam.
During the examination, each procedure was explained to the survey participant by the examiner (technicians, physician, phlebotomist, etc.). Survey participants had the opportunity to ask questions and refuse to participate in any procedures.
At the conclusion of their MEC appointment, participants were recruited to participate in post-MEC data collection activities. Eligibility for post-MEC data collection depended on the participant's age (at the time of screening) and gender and completion of the required components. In 2017-2018, a second 24-hour dietary interview was conducted after the MEC exam. Following the completion of the dietary interview, eligible persons were invited to complete the Flexible Consumer Behavior Survey (FCBS) supplemental module through a phone interview.
Automated Data Collection
Each MEC examination had computerized data collection with built-in data entry quality control checks. Unusual data entries were flagged, and a message was sent to the technicians to either verify or edit the response.
Translations of Protocol Instructions
All NHANES brochures, consent forms, hand cards, protocols, and correspondence were produced in English and Spanish. To facilitate the oversampling of the Asian population, which began in 2011, selected survey materials were translated into Mandarin Chinese, both traditional and simplified, Korean, and Vietnamese. Materials translated to these Asian languages included hand cards, glossaries of terms, consent forms, examination scripts, and informational brochures. All translations were done using the forward translation process approved by the NCHS Research Ethics Review Board. Staff participated in cultural competency training to help them recognize and respect cultural differences. Extensive training was completed with MEC staff to ensure the quality and comparability of staff interactions with non-English speaking respondents. Local interpreters were hired when necessary and were provided with translated materials and exam scripts to minimize errors in interpretation.
The qualifications for the staff, including the physician, dentist, phlebotomist, and health technicians are described in the component training manuals. All MEC staff completed the requirements for safety, subject privacy and confidentiality, and cardiopulmonary resuscitation training. In addition, all staff completed component-specific training to learn the standardized NHANES protocol for each of the examinations they performed.
Quality Control Monitoring
Quality control measures included equipment calibration and observations of examinations in the field. Site visits by NCHS staff, subject matter experts from collaborating agencies, and component-specific contract technical consultants were done throughout the year to monitor staff performance. Periodic retraining sessions were conducted with the MEC staff.
For a subset of examination components (e.g., body measurements), a reference examiner performed a second examination on selected participants during site visits to conduct gold standard assessments.
Data Preparation and Analytic Guidance
An extensive series of quality assurance and quality control analyses were completed for each examination component. Routine data preparation procedures included a review of frequency data, outliers, and technician notes. The MEC exam component manuals and component-specific documentations provided with released data files, describe the measures taken in greater detail. We strongly encourage data users to read all relevant documentation on the survey overall and for the specific data files to be used in their analysis. Specific data file documentation can be found via the link next to the respective data file on the NHANES website. Data users should also review the NHANES Analytic Guidelines prior to beginning any analyses.