# Overview of NHANES Survey Design and Weights

### Purpose

NHANES uses a complex sampling design and constructs sample weights to produce nationally representative data. Learning about the features of the NHANES survey design and weighting will help ensure that the results of your analyses represent unbiased estimates with accurate statistical significance levels. Much of the material presented in this module can also be found in the Continuous NHANES tutorial.

### Task 1: Explain NHANES Survey Design

NHANES data are **not obtained using a
simple random sample.** Rather, a complex, multistage,
probability sampling design is used to select participantâ€™s
representative of the civilian, non-institutionalized U.S.
population. Oversampling of certain population subgroups is
done to increase the reliability and precision of health
status indicator estimates for these groups. This task will describe the general statistical sampling used to identify NHANES participants.

### Warning

For
NHANES datasets, the use of sampling weights and sample
design variables is recommended for all analyses because the
sample design is a clustered design and incorporates
differential probabilities of selection. If you **fail to
account for the sampling parameters, you may obtain biased
estimates and overstate significance levels.**

### Task 2: Explain NHANES Sample Weights

NHANES has constructed various sample
weights for each two-year survey cycle to take into account
survey non-response, over-sampling, post-stratification, and
sampling error. This task describes how sample weights are
constructed in NHANES. Due to the way NHANES participants
are selected, **sample weights must be used** to produce
an unbiased national estimate. This task will also provide advice on the use of the sample weights when analyzing the data.

- Key Concepts about the NHANES Sample Weights
- When and How to Construct Weights When Combining Survey Cycles

### Task 3: Explain NHANES Environmental Chemical Data Subsample Weights

Some NHANES environmental analytes such as blood lead or blood mercury are obtained on a full sample of participants, therefore full sample MEC weights can be used for analysis. However, most environmental analytes are measured in 1/3 subsamples. For analysis of this subsample data, appropriate subsample weights must be used. They are included on any data file where relevant. This task will describe the special features of most environmental chemical sample weight.

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