Task 4: How to Identify Correct Sample Weights and File Locations

NHANES participants are asked to participate in a variety of survey components, some of which are completed with subsamples of examinees. Several laboratory assays, nutrition, environmental health assessments, and mental health interviews are completed on subamples of examinees. (Please see the respective survey protocol/documentation for more specific information. Each component subsample has its own designated sample weight, which accounts for the additional probability of selection into the subsample component, as well as the additional nonresponse.

Recall from the “Overview of NHANES Survey Design and Weights” module in the Survey Orientation course that the set of all individuals that have nonzero values for a particular version of sample weights comprise a nationally representative sample, so long as those sample weights are incorporated into statistical analyses.  However, the construction of sample weights does not take into account item non-response.  Item non-response is described in Module 9, Task 1: Key Concepts about Missing Data in NHANES. 

The table below shows the versions of sample weights that have been constructed for individuals within different groups of dietary data files. To produce estimates appropriately adjusted for survey non-response, it is important to check all of the variables in your analysis and select the weight variable that applies to all members of the smallest analysis subpopulation.  The following table lists the types of sample weights by decreasing sample size.

 

Sample Weights and Data Files

Sample Size Component Data file(s) Containing Sample Weight Variable Weight Variable
Largest Questionnaire, supplement use Demographic WTINT2YR
MEC Demographic WTMEC2YR
24-hour recall (Day 1) Individual Foods; Total Nutrient Intakes WTDRD1
24-hour recall (Day 1 and 2) Individual Foods; Total Nutrient Intakes WTDRD2
Smallest Special subsamples (e.g. fasting) Varies Varies, depending on the subsample

 

For example, if you wish to combine data from the Day 1 24-hour recall data and the supplement use section (collected during the household interview), you must restrict your analysis to individuals with dietary recall data.  To do this, you designate that persons with a non-zero dietary recall Day 1 sample weight value (variable WTDRD1 in the Individual Foods and Total Nutrient Intakes files) be included in the analysis. In doing so, you will be  analyzing the supplement use data for individuals who completed the first 24-hour recall.

Examples

Other examples for selecting the correct sampling weights for your analysis are included below:

All of the variables in the analysis were collected in the in-home interview

Some of the variables in the analysis were collected in the MEC

Some of the variables in this analysis were collected in the MEC, but have special circumstances

Some of the variables in the analysis were collected as part of a subsample

 

 

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