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 ;