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Frequently Asked Questions

Frequently Asked Questions

1. Where can I find the data files and list of data items that are available from NHANES 2011-2012?

2. Why are there so many data files?

3. Next to the name of each data files on the NHANES 2011-2012 data page, there is a "Doc File" link. Where does that link take you?

4. On the NHANES 2011-2012 data page I see links for data. How do I access the data from these links?

5. What format are the data files in? Can they be used with SAS, SPSS, or STATA?

6. Do I need to use SAS software to view NHANES data?

7. Have the NHANES data been released in different data formats?

8. When will other data be available from NHANES 2011-2012?

9. Where can I find the analytic guidelines (weighting, variance estimation, sample design)?

10. Will data and weights be available on public use files for single years such as 1999, 2000, 2001, or 2002?

11. Will data and weights be available on public use files in combined datasets for four-year, six-year, or eight-year periods such as 2001-2006, 2003-2008, or 2005-2012?

12. What is the sample size for a particular data item, questionnaire section, examination component, or laboratory analyte?

13. Where can I find a description of the codebook contents?

14. How do I determine the skip patterns for a questionnaire section?

15. How are missing values, "blank but applicable", "don't know" and other values coded?

16. I have questions about using the data, protocols, etc - where can I get help?

17. Why isn't the adolescent data on alcohol use, smoking, sexual behavior, reproductive health and drug use available as a public release file?

18. Are there variables which can identify whether survey participants are family members and/or live in the same household?

19. Can I identify what region of the country or what state or county a survey participant resides within?

20. I am interested in one or more questions which appear in the survey questionnaire but I cannot find the question in a codebook or data file available on your Internet site. What happened to it?



1.Where can I find the data files and list of data items that are available from NHANES 2011-2012?

Most of the NHANES 2011-2012 data are publically available and can be downloaded freely at the NHANES 2011-2012 data page. Click the links below for lists of data items that are currently available from NHANES 2011-2012:

2. Why are there so many data files?

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:

3.Next to the name of each data files on the NHANES 2011-2012 data page, there is a "Doc File" link. Where does that link take you?

Data File Name Doc File Data File Date Published
Demographic Variables and Sample Weights DEMO_G Doc Data [XPT - 2.54 MB ] September, 2013

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.

4. On the NHANES 2011-2012 data page I see links for data. How do I access the data from these links?

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).
The data files are released as SAS Transport files in .xpt format. They 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 NHANES 2011-2012 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.

5. What format are the data files in? Can they be used with SAS, SPSS, or STATA?

The data files are in SAS transport file 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 NHANES 2011-2012 is a complex probability sample and proper analysis of the data usually requires statistical software that specifically incorporates sample design complications such as weighting and clustering.

6.Do I need to use SAS software to view NHANES data?

No. You can view NHANES data with the SAS Universal Viewer—a free download from SAS Institute. Currently, most NHANES data 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.

7.Have the NHANES data been released in different data formats?

Since 1999, data files are released as SAS Transport files in .xpt format. This includes 1999+ survey data, as well as newly released or updated data files from NHANES III, II, and I. The SAS Transport 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. Also, the files can be opened with the free SAS Universal Viewer and converted to other formats for use with other software packages.

NHANES III data that were released or updated after 1999 are available as SAS Transport (.xpt) files and can be used like the continuous survey files. NHANES III data that were released before 1999 were released as .dat files, which are formatted ASCII data files (text files). Running the associated SAS code creates a SAS dataset. Please see the NHANES III Web Tutorial for instructions on using these data files. Additionally, the text files can be used with other software packages. Please see your software package's instructions for working with text files (.dat or .txt).

NHANES II and I data files released or updated after 1999 are also available as SAS Transport (.xpt files) and can be used like the continuous NHANES data files. NHANES II and I data files released before 1999 are formatted text files (.txt) wrapped in a self-extracting executable file (.exe). Running the associated SAS code creates a SAS dataset from the text file. Please see the NHANES II Web Tutorial or NHANES I Web Tutorial for instructions on how to do this. Additionally, the text files can be used with other software packages. Please see your software package's instructions for working with text files (.txt).

8. When will other data be available from NHANES 2011-2012?

As other data is processed and ready for public release it will be released on the NHANES website. Please check the website regularly and refer to the NHANES What's New page for details on the newly released or updated datasets. To receive e-mail announcements regarding NHANES activities, products, and release dates, please subscribe to the NHANES listserv.

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

9. Where can I find the analytic guidelines (weighting, variance estimation, sample design)?

The NHANES 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.

10.Will data and weights be available on public use files for single years such as 1999, 2000, 2001, or 2002?

No. Due to concerns regarding potential disclosure risk and analytic limitations of the annual sample, no single years of data in the continuous NHANES 1999+ will be publically release. Although the data are available at the NCHS Research Data Center , analyst needs to be aware that estimates for single year data are relatively unstable (have large variance estimates) since NHANES can only go to a small number of primary sampling units (PSUs) each year. Please refer to the Analytic Guidelines for more details.

11.Will data and weights be available on public use files in combined datasets forfour-year,six-year, or eight-year periods such as 2001-2006, 2003-2008, or 2005-2012?

No. Any two-year survey cycle may be combined with adjacent two-year releases to analyze data from multiple survey cycles. NCHS does not construct and include all possible weights for the combinations of multiple 2-year cycles in the public release files. Instead, NCHS supplies analysts with information on how to combine these cycles and construct the appropriate weights in the Analytic Guidelines available on the NHANES web site. When combining two or more 2-year cycles from 2001-2002 and onward, sample weights must be computed before beginning any analyses. For all data that includes 1999-2002, the 4-year weights provided by NCHS, must be used then include the additional weights for each 2-year cycle added. The rules for combining surveys also apply to subsamples. Please refer to the NHANES Analytic Guidelines to determine the appropriate methodology for analyses of combined years of data. The Continuous NHANES Web Tutorial module on Weighting also provides guidance.

Below are some brief examples for constructing the appropriate weights:

Six-year sample weights for NHANES 1999-2004 should be calculated by researchers as follows: With the first two dataset weights (NHANES 1999-2002) already averaged as a four-year sample weight, then the six year weight would be WT99-04 = (2/3) x WT99-02 + (1/3) x WT03-04, where WT99-02 is the variable WTMEC4YR from the NHANES 2001-2002 demographic file dataset, and WT03-04 is the variable WTMEC2YR from the NHANES 2003-2004 demographic file dataset. Eight-year sample weights for NHANES 1999-2006 should be calculated similarly to calculating the six-year sample weight. WT99-06 = (1/2) x WT99-02 + (1/4) x WT03-04 + (1/4) x WT05-06, where WT05-06 is the variable WTMEC2YR from the NHANES 2005-2006 demographic file dataset.

Six-year sample weights for 2001-2006 can be combining by using the 2-year weights found in the demographic files. For example, WT01-06 = (1/3) x WT01-02 + (1/3) x WT03-04 + (1/3) x WT05-06.

12.What is the sample size for a particular data item, questionnaire section, examination component, or laboratory analyte?

For any particular questionnaire section, examination component or laboratory data file you will only find records for survey participants that were eligible. For example, suppose 6,000 people were eligible for an examination in the MEC and only 5,000 were eligible for the muscular strength component due to age restrictions. Of the 5,000 suppose only 4,500 participated in the examination; the other 500 either refused or did not have enough time to participate in the exam. The muscular strength data file would now have 5,000 records with 500 records having missing data. For further details refer to the "frequency" counts document for each of the data files.

13. Where can I find a description of the codebook contents?

Documentation Contents

14. How do I determine the skip patterns for a questionnaire section?

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.

15. How are missing values, "blank but applicable", "don't know" and other values coded?

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. There is no longer a specific code for those cases where the variable response is "blank but applicable"; for such cases the values are designated as missing values. For laboratory data there are special considerations. When a laboratory value was less than the lower limit of detection (LOD), a "fill" value based on the LOD was used instead of the sample value as the sample value was deemed "not detectable." An indicator variable taking value (0 or 1) is used to identify which values are real and which values are fill values. Please review the codebook document for each of the data files for item-specific information.

16. I have questions about using the data, protocols, etc - where can I get help?

First, and most important, refer to the questionnaire, exam component, or laboratory descriptions. If you need help beyond this you can email CDC-info and submit your question.

17. Why isn't the adolescent data on alcohol use, smoking, sexual behavior, reproductive health and drug use available as a public release file?

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

18. Are there variables which can identify whether survey participants are family members and/or live in the same household?

In continuous NHANES 1999+, there is no way 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 NHANES 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 NHANES public or non-public data to produce household or family level estimates.

19. Can I identify what region of the country or what state or county a survey participant resides within?

Geographic identifiers are available but only through the Research Data Center (RDC), in order to protect the confidentiality of our participants. A list of NHANES Geocode variables is available through the Limited Access Datasets page.

20. I am interested in one or more questions which appear in the survey questionnaire but I cannot find the question in a codebook or data file available on your Internet site. What happened to it?

It sometimes means that the data is not yet ready to be publicly released. Other times, the staff 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.

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