3. Why are there so many data files? The data files have
been separated to reduce theamount of time to
download data and documentation from the Internet, along with thegreater ease in
producing, editing, and validatingdata 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: NHANES data
merge code example
4.
On the
NHANES 2003-2004 data page
I see links for data. How do I access the data from these links?
The Docs and Procedures files are in Adobe PDF format, so
you should be able to view these directly in your browser, if configured
with Adobe Acrobat. A PDF file can be saved from this view using the
"File/Save As..." menu and specifying a location on your local computer or
network to store the file. Or you can right-click the file name directly
on the webpage and select "Save Target As..." from the popup box, then
specify a location to save the file on your computer.
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.)
5.
Next to the name of each questionnaire section, laboratory component, or exam
component on the NHANES
2003-2004 data page there are links that appear as follows:
[Data, Docs, Procedures].
What are these links for?
In previous years, files for each data section was made available as a
self-extracting zip file containing the codebook, documentation, file
frequencies, and SAS transport dataset. In addition, each of the
codebook, documentation, file frequencies, and SAS transport dataset were
made directly, individually accessible by a separate link, in brackets,
next to the data section name.
Starting with data years 2003-2004 the documentation, codebook, and
frequencies have been combined in a single Adobe PDF file, accessible from
the single Docs link. A new, direct link to Procedures is
also being provided for each data section on the data page. For exam
sections this will link to the examination procedures manual for that
section, and for questionnaire sections this will link to the
questionnaire instrument. Procedure manuals for laboratory sections are
also available but as there may be multiple documents for a given lab,
these will remain on a separate page, accessible by clicking the
Laboratory Procedures Manuals link at the top of the list of
laboratory data sections.
Because all data and documentation files are directly accessible,
self-extracting zip files will no longer be required.
6.
What format are the data files in? Can
they be used with SAS, SPSS, or STATA? The files are
in SAS transport file format. They
can be used with any package that supports this file format. For statistical/analytical packages that do not support SAS
transport file format, you need to convert the file to a different format
using an appropriate software package. Please note that NHANES 2003-2004 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.
8.
Where can I find the analytic guidelines (weighting,
variance estimation, sample design)? The analytic
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.
9.
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
data file would 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.
10.
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.
11.
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.
12.
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. A second option is to contact NCHS by using the
Contact Us online feature.
If
you need help beyond this you can pose your question to the NHANES
listserv – please note however that the NHANES program staff do not
routinely provide technical responses to questions posted to the listserv.
13.
When will other data be available from NHANES 2003-2004? As other data is
processed and ready for public release it will be released on the NHANES
website. Certain data will only be available at the NCHS
Research Data Center. The
RDC data consists of adolescent data (people less than 20 years old) such
as: youth conduct disorder, sexual behavior, drug use, alcohol use, and
CDISC. Please refer to the NHANES
What's New
page for further details.
14.
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.
15. Will data and weights be available on public use files for single
years such as 1999, 2000, 2001, or 2002? No.
16.
Will data and weights be available on public use files in combined
datasets for three year and six year periods such as 1999-2001, 2002-2004,
or 1999-2004?
No. The continuous NHANES will be grouped for two year periods for
public release (i.e. 1999-2000, 2001-2002, 2003-2004, etc.). Combining two
or more two-year periods is possible (i.e. 1999-2002, etc.). The two-year
sample weights should be used for NHANES 1999-2000, NHANES 2001-2002 and
NHANES 2003-2004 analyses, respectively. The four-year sample weights should
be used for combined analyses of NHANES 1999-2000 & NHANES 2001-2002 data.
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, then the six year 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. Please
refer to the NHANES Analytic Guidelines provided with the data release files
to determine the appropriate methodology for analyses of combined years of
data.
17. Do I need to use SAS software to view
NHANES data?
No. You can view NHANES data with the SAS System Viewer—a free download from
SAS Institute. Currently, most NHANES is available in the SAS transport
format (.xpt), which can be used in several statistical software programs,
including SUDAAN and SPSS. Users desiring alternate data formats can use the
SAS Viewer to convert the transport file into space-, tab-, or
comma-delimited text files for use in additional software programs, such as
Microsoft Excel.