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 identical 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 is open a total of 5 days per week; the non-operational days change on a rotating basis so that appointments can be scheduled on any day of the week. Two examination sessions are conducted daily. Participants are randomly assigned to exams in the morning session, or in the afternoon or evening sessions.
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 whether he or she completed the required components. Post-MEC exam data collections in 2011-2014 include physical activity monitor (PAM), second 24-hour dietary interview, and 24-hour urine collections (subsample in 2014).
Automated Data Collection
Each MEC examination had computerized data collection. The examination component data entry systems had built-in quality control checks. Unusual data entries were flagged and a message was sent to the technicians. The technicians were required to either verify the original entry or edit the response. At the end of each examination session, data were sent to a central survey database.
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, consents, examination scripts, and informational brochures. All translations were done using the Institutional Review Board (IRB) approved forward translation process. 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 as part of quality assurance. Periodic retraining sessions were conducted with the MEC staff.
For a subset of examination components (e.g., body measurements, blood pressure and the dental examination), a "gold standard" examiner performed a second examination during a site visit.
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 include 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 that all data users, prior to any analysis of the data, read all relevant documentations 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 reference the NHANES Analytic Guidelines prior to beginning any analyses.