Task 2b: How to Use SAS Survey Code to Specify Sampling Parameters in NHANES III

The code needed to specify sampling design parameters using SAS Survey procedures is described below. In this example, the SAS Survey procedure, proc surveymeans, is used and the name of the dataset is BP_analysis_Data. Proc surveymeans is being used as a generic example, but the strata, cluster and weight statements apply to all SAS Survey procedures.

 

Step 1: Use data statement

When using SAS Survey procedures, the input dataset must be identified. However, the dataset does not have to be presorted by the sample design variables as it does in SUDAAN. Rather, the design variables—strata and PSU—are specified in subsequent steps.   

 

Step 2: Use strata statement

The strata statement names the variables that form the strata. For the combined 6 years NHANES III the variable that identifies the sample strata is named sdpstra6.

 

Step 3: Use cluster statement

The cluster statement names the variables that identify the clusters in a clustered sample design such as NHANES III. Since there is also a strata statement needed in NHANES III, clusters are nested within the strata by SAS Survey procedures.

In NHANES III the variable that represents the sample clusters is named sdppsu6 (primary sampling units or PSUs).

 

Step 4: Use weight statement

In NHANES III, a sample weight is assigned to each sample participant.  The sample weight is a measure of the number of individuals in the target population that the sampled individual represents. Sample weights are needed to obtain unbiased estimates of population parameters when the sample participants are chosen with unequal probabilities. (See module on weighting for more details).

The weight statement in SAS Survey procedures is required for all NHANES analyses. It identifies the sample weight. In this example, the MEC + Home exam weight for 6 years of data (wtpfhx6) is used.

 

Summary: Sample SAS Survey Procedure specifying sampling design parameters

The following table shows how to combine the statements described above to properly specify the sample design parameters and sample weights using SAS Survey procedures. The procedure, proc surveymeans, is used as an example, but the strata, cluster and weight statements can be used in the same manner for all SAS Survey procedures. The steps in this task identify the most basic statements used in SAS Survey procedures to account for the complex sample design of NHANES III. Additional procedure options can be added to these statements to customize the variance estimates, statistics and the output from your procedure to suit individual analytic needs. Please consult the SAS/STAT manual for specifications on the options for each SAS Survey procedure.

 

SAS surveymeans Procedure
Statements Explanation
proc surveymeans data= BP_analysis_Data;

Use the SAS Survey procedure, proc surveymeans, to calculate means and standard errors, and specify the data set (BP_analysis_Data).
stratum sdpstra6;

Use the stratum statement to specify the strata (sdpstra6) -- this accounts for the design effects of stratification.

cluster sdppsu6;

Use the cluster statement to specify primary sampling unit (sdppsu6) — this accounts for the design effects of clustering.

weight wtpfhx6;

Use the weight statement to account for the unequal probability of sampling, survey nonresponse and adjustments to population control totals. In this example, the MEC + home exam weight for 6 years of data is used (wtpfhx6).

Reference: SAS Institute Inc., SAS/STAT User's Guide, Version 9.1; see: Survey Means Procedure