Table of Contents

Component Description

This release provides raw ambient light data from the physical activity monitor (PAM) component for the NHANES National Youth Fitness Survey (NNYFS). Participants were asked to wear the PAM for seven consecutive days to collect objective information on 24-hour movement when awake and asleep. The same PAM component was also included in the main NHANES program in 2011-2014.

Measurement of physical activity with accelerometers had been implemented in NHANES 2003-2006. The PAM used in 2012 NNYFS and NHANES 2011-2014 measured acceleration 80 times a second (80 Hz) and ambient light (lux) once a second (1 Hz). The collected measurements have been summarized over each minute, hour, and day for each participant and released in separate files for the NNYFS (i.e., Y_PAXHD, Y_PAXMIN, Y_PAXHR, Y_PAXDAY). This release provides the raw lux data collected through the PAM component.

Lux is the International System of Units (SI) derived unit of illuminance. One lux is equal to one lumen per square meter. Ambient light, measured here in lux, may affect subject sleeping habits and could be a useful tool in analyzing circadian rhythms and sleeping patterns. Some estimates of lux values are shown in the following table (ActiGraph, 2013). NOTE: These levels are only estimates and are not meant for exact interpretation of the light detected by the device. Lux values reported by the PAM devices are only intended as a general guideline for average light intensities.

Lux Level Interpretation Comparison
1 Twilight
50 Family living room
100 Very dark overcast day
320-500 Office lighting
400 Sunrise/sunset
1,000 Overcast day
10,000-25,000 Full daylight

Datasets included in this release were comma-separated-value (.csv) text files compressed using the bzip2 format to reduce file sizes. There is one zipped archive file (tar.bz2 format) for each participant. As bzip2 is a freely available open-source file compression program, these zipped archive files can be uncompressed using standard compression software (e.g., Winzip) or with common compression utilities available in R and Python.

Eligible Sample

All participants were asked to wear a PAM for 7 consecutive days.

Protocol and Procedure

Participants were asked to wear the PAM starting on the day of their exam in the NHANES Mobile Examination Center (MEC), to keep wearing the PAM 24 hours a day for the next 7 days, and to remove it on the morning of the 9th day. The device used in NHANES was the ActiGraph model GT3X+, manufactured by ActiGraph of Pensacola, FL (ActiGraph, 2013). The devices were programmed to begin detecting and recording acceleration and ambient light at the end of the participant’s MEC exam session and to stop recording data eight days later (i.e., on the 9th calendar day). The device was placed on a mesh wristband (like a watch), custom fitted for each participant’s wrist and worn on the non-dominant hand, if possible. The participant was told not to do anything with the device except to wear it. If the participant needed to remove it, for any reason, he/she was instructed to put it back on the same wrist in the same orientation, as soon as possible.

A toll-free number and information materials describing the PAM were given to the participant. No activity logs, diaries, or records were kept by the participant. The PAMs were returned by mail in postage-paid padded envelopes provided to each participant.

For further details see the PAM procedures manual.

Quality Assurance & Quality Control

Survey staff completed an examiner training program that covered the basic operating features of the PAM, equipment operation, subject recruitment strategies, PAM device initialization, and troubleshooting. Field performance and response rates were monitored by NCHS and contractor staff.

Data Processing and Editing

Not every eligible participant has a data file. For example, if a participant refused to wear the PAM, did not return it, or the data could not be retrieved from a damaged PAM, then the participant does not have a data file. Per the protocol, the first day and last day of data collection for each participant are partial days.

The lux files (one per participant) use the following naming convention:


where SEQN is the respondent sequence number of the participant.

The lux file for each participant contains the following two fields:

HEADER_TIMESTAMP (Date and time stamp)
A datetime in the following format: YYYY-MM-DD HH:MM:SS, where YYYY-MM-DD represents a date (e.g., “2000-01-03”) and HH:MM:SS represents a time.

LUX (Lux value)
Measure of ambient light in lux (i.e., lm/m2; lumen per square meter). Values at or above 2,500 lux were coded as “2,500 lux”.

Analytic Notes

The NNYFS examination sample weights should be used to analyze the PAM data. Please refer to the NNYFS Analytic Guidelines for details on the use of sample weights and other analytic issues.

The PAM component was reintroduced to NHANES in 2011-2014 and was included in the 2012 NHANES National Youth Fitness Survey (NNYFS) as well. A total of 16,417 persons ages 3 years and older wore the PAM during these surveys and had data available for analysis: 2011-2012 NHANES participants 6 years and older (n=6,917); 2013-14 NHANES participants 3 years and older (n=7,776); and 2012 NNYFS participants 3-15 years (1,477). In addition, PAM data were collected for 3-5 years old participants in the 2012 NHANES (n=247) as well. However, due to disclosure concerns, these data are only available in the NCHS Research Data Center (RDC). Overall, 96% of participants with data wore the PAM until the 9th day. About 2% of the participants with PAM data wore it for less than 7 days. On the non-partial days (that is, not the first or last days of wear) the mean number of valid minutes per day was ~1,437 minutes out of a possible 1,440 valid minute.

In the NNYFS, PAM data is available for 1,477 participants 3 years and older. The current release (Y_PAXLUX) consists of 1,477 zipped archive files – one for each participant. Due to the quantity of the data, these files are stored at the CDC File Transfer Protocol (FTP) Server. To access, please click on the “Y_PAXLUX Data” link at the NNYFS Examination Data webpage and follow the instructions on the screen to download the files.