Accounting for the complex
sampling design of NHANES II 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 procedures 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 II 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 II."

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 procedures, 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 procedures also specify 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 II."

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 II, reflecting the unequal probability of selection, nonresponse adjustment, and adjustment to independent population control totals. When unequal selection probability is applied, as in the NHANES II 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, MVUs were not created for NHANES II. Instead, PSEUDO primary sampling units and stratification variables are provided. In NHANES II, 32 pseudo-strata and 64 pseudo-PSUs were created for variance estimation.