Step 1: When to construct weights for combined survey cycles
Sample weights for NHANES 19992000 were based on population estimates developed by the Bureau of the Census before the Year 2000 Decennial Census counts became available. The 2year sample weights for NHANES 20012002, and all other subsequent 2year cycles, are based on population estimates that incorporate the year 2000 Census counts. Because different population bases were used, the 2year weights for 19992000 and 20012002 are not directly comparable. Therefore, when combining 19992000 with 20012002 survey years in analyses, you must use the 4year sample weights provided by NCHS since these have been created to account for the two different reference populations.
For both 19992000 and 20012002 survey cycles, the demographic file contains the weight variables
wtint2yr
and wtint4yr
for all interviewed sample persons,
wtmec2yr
and wtmec4yr
for the sample persons who have MEC data items, and
 twoyear and fouryear (for subsample datasets with consistent data elements across two survey cycles) subsample weights for selected sample persons.
Example 1: How to combine those MEC examined for 19992000 with 20012002
You must use the 4 year weights provided (wtmec4yr
) in the SAS demographic file (see explanation above).
Because NHANES 20032004 and all future survey cycles use the same year 2000 Census counts that were used for NHANES 20012002, NCHS does not need to create special 4year weights. NCHS does not construct and include all possible weights for the combinations of multiple 2year cycles in the public release files because it would be impractical to do so. Instead, NCHS supplies analysts with information on how to combine these cycles and construct the appropriate weights.
Step 2: How to construct weights for combined survey cycles for NHANES 20012002 and beyond
When you combine two or more 2year cycles of the continuous NHANES for NHANES 20012002 and beyond, you must construct sample weights before beginning any analyses. When survey cycles are combined, the estimates will be representative of the population at the midpoint of the combined survey period. When you construct weights appropriately, as described in the next step, you will rescale the weights so that the sum of the weights match the survey population at the midpoint of that period.
In order to construct 4 year, 6 year 8 year, etc weights for survey cycles of the continuous NHANES for NHANES 20012002 and beyond, the following formulae should be used:
Formulae for Constructing Weights for NHANES 20012002 and Beyond**
Number of Survey Years Used 
Survey Cycle Code* 
Code with Formula for Combining Weights across Survey Cycles 
4 years

If sddsrvyr in (2,3) then
If sddsrvyr in (3,4) then
If sddsrvyr in (4,5) then

MEC4YR = 1/2 * WTMEC2YR; /*for 20012004*/
MEC4YR = 1/2 * WTMEC2YR; /*for 20032006*/
MEC4YR = 1/2 * WTMEC2YR; /*for 20052008*/

6 years

If sddsrvyr in (2,3,4) then
If sddsrvyr in (3,4,5) then

MEC6YR = 1/3 * WTMEC2YR; /*for 20012006*/
MEC6YR = 1/3 * WTMEC2YR; /*for 20032008*/

8 years

If sddsrvyr in (2,3,4,5) then

MEC8YR = 1/4 * WTMEC2YR; /*for 20012008*/

*SDDSRVYR
is the survey cycle variable, i.e.
1 = 19992000
2 = 20012002
3 = 20032004
4 = 20052006
5 = 20072008
Etc.
** To construct weights across survey cycles that include the 19992000 survey cycle, see the examples below.
Examples for Constructing 4year, 6year, 8year, etc... Weights for Survey Cycles of the Continuous NHANES (Including Combinations with NHANES 19992000):
4 years of data from 20012004
For 4 years of data from 20012004 a weight should be constructed as:
if sddsrvyr=2 or sddsrvyr=3 then
MEC4YR = 1/2 * WTMEC2YR ;
Example 2: How to combine 4 years of data from 20012002 with 20032004
ANSWER: As stated above, create a weight variable by combining the 2year weights (WTMEC2YR
) found in the SAS demographic file using the following code:
if sddsrvyr=2 or sddsrvyr=3 then
MEC4YR = 1/2 * WTMEC2YR ;
6 years of data from 19992004
For 6 years of data from 19992004 a weight should be constructed as:
if sddsrvyr=1 or sddsrvyr=2 then
MEC6YR = 2/3* WTMEC4YR ; /* for 19992002 */
If sddsrvyr=3 then
MEC6YR = 1/3 * WTMEC2YR ; /* for 20032004 */
WARNING
For all data that includes 19992002, you must use the 4 year weights provided by NCHS, then include the additional weights for each 2year cycle added.
Example 3: How to combine 6 years of data from 19992004
ANSWER: Because you are using the 19992000 survey combined with 20012002, you must begin with the 4year weights provided (WTMEC4YR
) in conjunction with the 2year weights for 20032004 (WTMEC2YR
) found in the SAS demographic files. This will allow you to create a 6year weight variable (MEC6YR
) using the following code.
For 6 years of data from 19992004 a weight should be constructed as:
if sddsrvyr=1 or sddsrvyr=2 then
MEC6YR = 2/3 * WTMEC4YR ; /* for 19992002 */
If sddsrvyr=3 then
MEC6YR = 1/3 * WTMEC2YR ; /* for 20032004 */
6 years of data from 20012006
For 6 years of data from 20012006 a weight should be constructed as:
if sddsrvyr in (2,3,4) then
MEC6YR = 1/3 * WTMEC2YR;
Example 4: How to combine multiple 2year weights that do not include 1999/2000 to make 6 or more years of data (i.e., 2001/2002, 2003/2004, 2005/2006).
ANSWER: Because you are NOT using any data from 19992000 you can combine all 2year weights (WTMEC2YR
) found in the SAS demographic files as follows to create another 6year weight variable MEC6YR
using the following code:
For 6 years of data from 20012006 a weight should be constructed as:
if sddsrvyr in (2,3,4) then
MEC6YR = 1/3 * WTMEC2YR;
Similarly, for 8 years of data from 20012008 and beyond, you would combine the 2year weights (WTMEC2YR
) using the correct proportion of each. For example:
For 8 years of data from 20012008 a weight should be constructed as:
if sddsrvyr in (2,3,4,5) then
MEC8YR = 1/4 * WTMEC2YR;
8 years of data from 19992006
For 8 years of data from 19992006 a weight should be constructed as:
if sddsrvyr=1 or sddsrvyr=2 then
MEC8YR = 1/2 * WTMEC4YR ; /* for 19992002 */
if sddsrvyr=3 or sddsrvyr=4 then
MEC8YR = 1/4 * WTMEC2YR ; /* for 20032006 */
WARNING
For all data that includes 19992002, you must use the 4 year weights provided by NCHS, then include the additional weights for each 2year cycle added.
Example 5: How to combine 8 years of data that include 19992000 through 20052006
ANSWER: You must use the 4year weights provided for 19992002 (WTMEC4YR
) with the 2year weights for both 20032004 and 20052006 (WTMEC2YR
) to create a 8year weight variable (MEC8YR
).
For 8 years of data from 19992006 a weight should be constructed as:
if sddsrvyr=1 or sddsrvyr=2 then
MEC8YR = 1/2 * WTMEC4YR ; /* for 19992002 */
if sddsrvyr=3 or sddsrvyr=4 then
MEC8YR = 1/4 * WTMEC2YR ; /* for 20032006 */
Again, future years of data can continue to be added using the same methods as above for combining cycles by taking the correct proportion of the 4year and 2year weights.
10 years of data from 19992008
For 10 years of data from 19992008 a weight should be constructed as:
if sddsrvyr=1 or sddsrvyr=2 then
MEC10YR = 2/5 * WTMEC4YR ; /* for 19992002 */
if sddsrvyr=3 or sddsrvyr=4 or sddsrvyr=5 then
MEC10YR = 1/5 * WTMEC2YR ; /* for 20032008 */
Example 6: How to combine 10 years of data that include 19992000 through 20072008
ANSWER: You must use the 4year weights provided for 19992002 (WTMEC4YR
) with the 2year weights for 20032004, 20052006 and 20072008 (WTMEC2YR
) to create a 10year weight variable (MEC10YR
).
For 10 years of data from 19992008 a weight should be constructed as:
if sddsrvyr=1 or sddsrvyr=2 then
MEC10YR = 2/5 * WTMEC4YR ; /* for 19992002 */
if sddsrvyr=3 or sddsrvyr=4 or sddsrvyr=5 then
MEC10YR = 1/5 * WTMEC2YR ; /* for 20032008 */
Again, future years of data can continue to be added using the same methods as above for combining cycles by taking the correct proportion of the 4year and 2year weights.