NNYFS 2012 Frequently Asked Questions (FAQ)

The NHANES National Youth Fitness Survey (NNYFS) was conducted in 2012 as a one-time survey to collect data on physical activity and fitness levels in order to provide an evaluation of the health and fitness of children and teens in the U.S. ages 3 to 15 years. The NNYFS collected data on physical activity and fitness levels through interviews and fitness tests. The survey was conducted in 2012 by the Division of Health and Nutrition Examination Surveys, National Center for Health Statistics, part of the Centers for Disease Control and Prevention.

The NNYFS was a nationally representative survey of the non-institutionalized, resident population ages 3 to 15 years. The design was similar to that of the continuous NHANES, but with no oversampling of any race-ethnicity subgroups. A total of 1,640 youth were interviewed for the NNYFS, and 1,576 youth were examined in the NNYFS in 2012.

More information on the NNYFS sample design can be found in the NHANES National Youth Fitness Survey (NNYFS) Analytic Guidelines available on the NNYFS website.

The NNYFS consisted of a home interview and a physical fitness examination in a Mobile Examination Center (MEC). For most of the questionnaires, an adult family member, aged 18 years or older, was interviewed as a proxy for the survey participant. The only exception during the home interview was for the Physical Activity and Physical Fitness (PAQ) questionnaire in which respondents’ ages 12-15 years answered the questions themselves. At the MEC, participants’ ages 12-15 years answered for themselves questions on drug use, alcohol use and smoking and tobacco use. For the dietary interview, children ages 3-5 years were interviewed by proxy, children ages 6-11 years were proxy-assisted and youth ages 12+ responded for themselves. Please see Survey Contents page for a list of questionnaires and examinations conducted during the NNYFS.

The NHANES National Youth Fitness Survey (NNYFS) plan and operations report provides detailed information on how the Division of Health and Nutrition Examination Surveys (DHANES) of the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention (CDC), planned, implemented, and conducted the NNYFS in 2012.

• NNYFS examination manuals
• NNYFS questionnaire instruments

Most of the NNYFS data are publically available and can be downloaded freely at the NNYFS data page. Click the links below for lists of data items that are currently available from NNYFS:

• List/description of demographic data items
• List/description of examination data items
• List/description of questionnaire data items
• List/description of dietary data items
• List/description of limited access data items

The data files have been separated to reduce the amount of time to download data and documentation from the Internet, along with the greater ease in producing, editing, and validating data files. This does require that you merge files together for analysis. Please refer to the following SAS code example to learn how to merge files together (Using the demographic and body measures files as an example):

LIBNAME DM XPORT ‘C:My FilesTempY_demo.xpt’;
LIBNAME EX XPORT ‘C:My FilesTempY_BMX.xpt’;
LIBNAME OUT ‘C:My FilesTempNHANES’;

DATA OUT.BMX_DEMO;
MERGE DM.Y_DEMO (IN=A)
EX.Y_BMX;
BY SEQN;
IF A;
RUN;

The link allows you to view the documentation, including the codebook with the frequency distribution for each item in a particular data file. This can be used to verify the sample size for any particular data item. The documentation is formatted as a webpage so you should be able to view these directly in your browser.

Clicking on the Data link will open a dialog box from which you can specify a location to store the file (using the “Save” button) or open it directly with SAS (using the “Open” button.)

NNYFS data files are released as SAS Transport files in .xpt format, which can be used in several statistical software programs, including SAS, SUDAAN, SPSS, and, STATA. The files can be opened directly as a temporary work file or permanent libraries can be created using SAS. Please see the Continuous NHANES Web Tutorial for instructions.

Users desiring alternate data formats can use the free SAS Universal Viewer to convert the transport file into a comma-delimited text file (.csv) for use in additional software programs. Please note that NNYFS is a complex probability sample and proper analysis of the data requires statistical software that specifically incorporates sample design complications such as weighting and clustering.

NNYFS data files are released as SAS Transport files in .xpt format. In additional to SAS, they can also be used with any package that supports this file format, such as SUDAAN, SPSS, and STATA. For statistical/analytical packages that do not support SAS transport file format, you can convert the file to a different format using the free SAS Universal Viewer or other appropriate software package.

Please note that NNYFS is a complex probability sample and proper analysis of the data requires statistical software that specifically incorporates sample design complications such as weighting and clustering.

No. You can view NNYFS data with the SAS Universal Viewer—a free download from SAS Institute. Currently, NNYFS is available in the SAS transport format (.xpt), which can be used in several statistical software programs, including SUDAAN, STATA, and SPSS. Users desiring alternate data formats can use the SAS Viewer to convert the transport file into a comma-delimited text file (.csv) for use in additional software programs, such as Microsoft Excel.

Learn more about SAS Universal Viewer.

As other data is processed and ready for public release it will be released on the NNYFS website. Please check the website regularly and refer to the NNYFS What’s New page for details on the newly released or updated datasets.

Note that certain data will only be available at the NCHS Research Data Center. Documentation for some of these datasets is available on the Limited Access Datasets page.

To receive e-mail announcements regarding NHANES activities, products, and release dates, please subscribe to the NHANES listserv.

The NNYFS analytic guidelines are available on the website. The guidelines provide information on the sample design and on recommended methodologies for analyzing the data. In particular, the guidelines provide information on how the sample persons were selected, how the various survey weights were calculated, what particular survey weight should be used to provide survey estimates, how to compute sampling variances for those estimates, and recommended sample sizes for analysis.

For any particular questionnaire section or examination component data file you will only find records for survey participants that were eligible. For example, suppose 1,576 people were eligible for an examination in the MEC and only 506 were eligible for the Cardiovascular Fitness component due to age restrictions. Of the 506 suppose only 475 participated in the examination; the other 31 either refused or did not have enough time to participate in the exam. The Cardiovascular Fitness data file would have 506 records with 31 records having missing data. For further details refer to the “frequency” counts document for each of the data files.

The first step is to review all of the documentation for the questionnaires. To review skip patterns look at the complete questionnaire instrument. Please note that not all questionnaire items are released due to small sample sizes and confidentiality/sensitivity issues, but all skip pattern integrity was maintained and validated.

There are codes for refused (7-fill: that is 7, or 77, or 777, …, depending on the number of digits required for a particular data value), don’t know (9-fill), and missing values (a blank field) which means the person was not asked the question or given the test. Please review the codebook document for each of the data files for item-specific information.

First, and most important, refer to the questionnaire instruments or examination manuals. If you need help beyond this you can email CDC-info and submit your question.

These files have not been released on the NNYFS website due to confidentiality concerns. Adolescent data files containing this sensitive information will be made available at the NCHS Research Data Center.

In NNYFS, you would not be able to identify whether one or more survey participants are related or live in the same household based on publically released data. Only limited information on family and household members’ relationship was collected in the survey, but this information as well as the Information identifying whether the participants live in the same household is only available through the Research Data Center. However, please note that NNYFS was not designed to produce estimates at the household or family level, only at the person-level. As such, it is not recommended that analysts use the NNYFS public or non-public data to produce household or family level estimates.

Geographic identifiers are only available through the NCHS Research Data Center (RDC), in order to protect the confidentiality of our participants.

It sometimes means that the data is not yet ready to be publicly released. Other times, staff may have determined that a question poses a risk of disclosure to our survey participants. Under these circumstances the data are made available only through the Research Data Center. Documentation for some of these datasets is available on the Limited Access Datasets page. Please send an email to cdcinfo@cdc.gov to inquire about the status of a particular question.

The NNYFS consisted of a home interview and a physical fitness examination in a Mobile Examination Center (MEC). Blood and urine was not collected from NNYFS participants.

Data from NNYFS can be combined with NHANES. For more information, see the NNYFS analytic guidelines . Combined weight for NNYFS and NHANES 2012 (1-year weights) are available but only through the Research Data Center (RDC).