Task 2: When and How to Construct Weights When Combining Survey Cycles

Step 1: When to construct weights for combined survey cycles

Sample weights for NHANES 1999-2000 were based on population estimates developed by the Bureau of the Census before the Year 2000 Decennial Census counts became available. The 2-year sample weights for NHANES 2001-2002, and all other subsequent 2-year cycles, are based on population estimates that incorporate the year 2000 Census counts. Because different population bases were used, the 2-year weights for 1999-2000 and 2001-2002 are not directly comparable. Therefore, when combining 1999-2000 with 2001-2002 survey years in analyses, you must use the 4-year sample weights provided by NCHS since these have been created to account for the two different reference populations.

For both 1999-2000 and 2001-2002 survey cycles, the demographic file contains the weight variables

How to combine those MEC examined for 1999-2000 with 2001-2002

You must use the 4 year weights provided (wtmec4yr) in the SAS demographic file (see explanation above).

Because NHANES 2003-2004 and all future survey cycles use the same year 2000 Census counts that were used for NHANES 2001-2002, NCHS does not need to create special 4-year weights. NCHS does not construct and include all possible weights for the combinations of multiple 2-year 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. 

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Step 2: How to construct weights for combined survey cycles for NHANES 2001-2002 and beyond

When you combine two or more 2-year cycles of the continuous NHANES for NHANES 2001-2002 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 2001-2002 and beyond, the following formulae should be used:

Formulae for Constructing Weights for NHANES 2001-2002 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 2001-2004*/

MEC4YR = 1/2 * WTMEC2YR; /*for 2003-2006*/

MEC4YR = 1/2 * WTMEC2YR; /*for 2005-2008*/

6 years

If sddsrvyr in (2,3,4) then

If sddsrvyr in (3,4,5) then

MEC6YR = 1/3 * WTMEC2YR; /*for 2001-2006*/

MEC6YR = 1/3 * WTMEC2YR; /*for 2003-2008*/

8 years

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

MEC8YR = 1/4 * WTMEC2YR; /*for 2001-2008*/

*SDDSRVYR is the survey cycle variable, i.e.
1 = 1999-2000
2 = 2001-2002
3 = 2003-2004
4 = 2005-2006
5 = 2007-2008
Etc.

** To construct weights across survey cycles that include the 1999-2000 survey cycle, see the examples below.

Examples for Constructing 4-year, 6-year, 8-year, etc... Weights for Survey Cycles of the Continuous NHANES (Including Combinations with NHANES 1999-2000):

4 years of data from 2001-2004

For 4 years of data from 2001-2004 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 2001-2002 with 2003-2004

Answer:  As stated above, create a weight variable by combining the 2-year weights (WTMEC2YR) found in the SAS demographic file using the following code:

if sddsrvyr=2 or sddsrvyr=3 then
    MEC4YR  =  1/2 * WTMEC2YR ; 

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6 years of data from 1999-2004

For 6 years of data from 1999-2004 a weight should be constructed as:

if sddsrvyr=1 or  sddsrvyr=2 then
    MEC6YR  = 2/3*  WTMEC4YR ; /* for 1999-2002 */

If sddsrvyr=3  then
    MEC6YR = 1/3 * WTMEC2YR  ; /* for 2003-2004 */

warning iconWARNING

For all data that includes 1999-2002, you must use the 4 year weights provided by NCHS, then include the additional weights for each 2-year cycle added.

Example 3: How to combine 6 years of data from 1999-2004

Answer: Because you are using the 1999-2000 survey combined with 2001-2002, you must begin with the 4-year weights provided (WTMEC4YR) in conjunction with the 2-year weights for 2003-2004 (WTMEC2YR) found in the SAS demographic files. This will allow you to create a 6-year weight variable (MEC6YR) using the following code.

For 6 years of data from 1999-2004 a weight should be constructed as:

if sddsrvyr=1 or  sddsrvyr=2 then
    MEC6YR  = 2/3 *  WTMEC4YR ; /* for 1999-2002 */

If sddsrvyr=3  then
    MEC6YR = 1/3 * WTMEC2YR  ; /* for 2003-2004 */

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6 years of data from 2001-2006

For 6 years of data from 2001-2006 a weight should be constructed as:

if sddsrvyr in (2,3,4)  then
    MEC6YR   =  1/3 * WTMEC2YR;

Example 4:  How to combine multiple 2-year 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 1999-2000 you can combine all 2-year weights (WTMEC2YR) found in the SAS demographic files as follows to create another 6-year weight variable MEC6YR using the following code:

For 6 years of data from 2001-2006 a weight should be constructed as:

if sddsrvyr in (2,3,4)  then
    MEC6YR   =  1/3 * WTMEC2YR;

Similarly, for 8 years of data from 2001-2008 and beyond,  you would combine the 2-year weights (WTMEC2YR) using the correct proportion of each.  For example:

For 8 years of data from 2001-2008 a weight should be constructed as:

if sddsrvyr in (2,3,4,5)  then
    MEC8YR   =  1/4 * WTMEC2YR;

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8 years of data from 1999-2006

For 8 years of data from 1999-2006 a weight should be constructed as:

if sddsrvyr=1 or sddsrvyr=2 then
    MEC8YR  = 1/2 * WTMEC4YR ; /* for 1999-2002 */

if sddsrvyr=3 or sddsrvyr=4 then
    MEC8YR = 1/4 * WTMEC2YR ; /* for 2003-2006  */

warning iconWARNING

For all data that includes 1999-2002, you must use the 4 year weights provided by NCHS, then include the additional weights for each 2-year cycle added.

Example 5: How to combine 8 years of data that include 1999-2000 through 2005-2006

Answer:  You must use the 4-year weights provided for 1999-2002 (WTMEC4YR)  with the 2-year weights for both 2003-2004 and 2005-2006 (WTMEC2YR) to create a 8-year weight variable (MEC8YR)..

For 8 years of data from 1999-2006 a weight should be constructed as:

if sddsrvyr=1 or sddsrvyr=2 then
    MEC8YR  = 1/2 * WTMEC4YR ; /* for 1999-2002 */

if sddsrvyr=3 or sddsrvyr=4 then
    MEC8YR = 1/4 * WTMEC2YR ; /* for 2003-2006  */

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 4-year and 2-year weights. 

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10 years of data from 1999-2008

For 10 years of data from 1999-2008 a weight should be constructed as:

if sddsrvyr=1 or sddsrvyr=2 then
    MEC10YR  = 2/5 * WTMEC4YR ; /* for 1999-2002 */

if sddsrvyr=3 or sddsrvyr=4 or sddsrvyr=5 then
    MEC10YR = 1/5 * WTMEC2YR ; /* for 2003-2008  */

Example 6: How to combine 10 years of data that include 1999-2000 through 2007-2008

Answer:  You must use the 4-year weights provided for 1999-2002 (WTMEC4YR)  with the 2-year weights for 2003-2004, 2005-2006 and 2007-2008 (WTMEC2YR) to create a 10-year weight variable (MEC10YR)..

For 10 years of data from 1999-2008 a weight should be constructed as:

if sddsrvyr=1 or sddsrvyr=2 then
    MEC10YR  = 2/5 * WTMEC4YR ; /* for 1999-2002 */

if sddsrvyr=3 or sddsrvyr=4 or sddsrvyr=5 then
    MEC10YR = 1/5 * WTMEC2YR ; /* for 2003-2008  */

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 4-year and 2-year weights. 

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