Accounting for
the complex sampling design of NHANES III is **critical when calculating
statistical estimates and estimating standard errors** of means, geometric
means, percentages and other statistics. Replication and linearization are two
statistical methods that can be used to properly address these complex design
issues. SAS Survey and SUDAAN use linearization for calculating standard errors
for a variety of statistics, such as means, geometric means and percentages.

Currently, SUDAAN offers six options for designating survey design (see SUDAAN manual for more details about the use and implications of all design options). SUDAAN assumes a with replacement (WR) design if the design parameter is omitted.

In the next task, you will be using the **with replacement (WR)** design for
analyzing NHANES III data.

** **In order to implement the WR sampling option in
SUDAAN, design variables specifying the first stage of the cluster design and
the sample weight are needed.

For more detailed information and sample code, see "Task 2a: How to Use SUDAAN Code to Specify Sampling Parameters in NHANES III."

In SAS, a group of procedures, known as the Survey procedures, produce
estimates from complex sample survey data. These procedures can also produce
variance estimates through linearization (see variance estimation module)
and confidence limits on many estimates. Currently, Taylor Series
Linearization is the only variance estimation method available through SAS
Survey procedures. In the SAS Survey procedure, the sample design is not
directly specified in the *proc* statement, as in SUDAAN, but rather, strata and PSU
variables are specified in separate statements. Similarly, SAS Survey procedure
also specifies the *weight* statement. For more detailed information and
sample code, see "Task 2b: How to Use SAS Survey Code to Specify Sampling
Parameters in NHANES III."

A sample weight is assigned to each sample person. It is a measure of the number of people in the population represented by that sample person in NHANES III, reflecting the unequal probability of selection, nonresponse adjustment, and adjustment to independent population controls. When unequal selection probability is applied, as in the NHANES III sample, the sample weights are used to produce an unbiased national estimate. More information about sample weights and how they are created can be found in the Weighting module.

Unlike in continuous NHANES where masked variance units (MVUs) were used, NHANES III did not create MVUs, Instead, PSEUDO primary sampling units and stratification variables are provided. In NHANES III, 49 pseudo strata and 98 pseudo-PSUs were created for variance estimation.