Specifying the Sample Design |

PROC SURVEYFREQ produces tables and statistics that are based on the sample design used to obtain the survey data. PROC SURVEYFREQ can be used for single-stage or multistage designs, with or without stratification, and with or without unequal weighting. To analyze your survey data with PROC SURVEYFREQ, you need to provide sample design information for the procedure. This information can include design strata, clusters, and sampling weights. You can provide sample design information with the STRATA, CLUSTER, and WEIGHT statements, and with the RATE= or TOTAL= option in the PROC SURVEYFREQ statement.

If you provide replicate weights for BRR or jackknife variance estimation, you do not need to specify a STRATA or CLUSTER statement. Otherwise, you should specify STRATA and CLUSTER statements whenever your design includes stratification and clustering.

When there are clusters (PSUs) in the sample design, the procedure estimates variance by using the PSUs, as described in the section Statistical Computations. For a multistage sample design, the variance estimation depends only on the first stage of the sample design. Therefore, the required input includes only first-stage cluster (PSU) and first-stage stratum identification. You do not need to input design information about any additional stages of sampling.

If your sample design is stratified at the first stage of sampling, use the STRATA statement to name the variables that form the strata. The combinations of categories of STRATA variables define the strata in the sample, where strata are nonoverlapping subgroups that were sampled independently. If your sample design has stratification at multiple stages, you should identify only the first-stage strata in the STRATA statement.

If you use a REPWEIGHTS statement to provide replicate weights for BRR or jackknife variance estimation, you do not need to specify a STRATA statement. Otherwise, you should specify a STRATA statement whenever your design includes stratification. If you do not specify a STRATA statement or a REPWEIGHTS statement, then PROC SURVEYFREQ assumes there is no stratification at the first stage.

If your sample design selects clusters at the first stage of sampling, use the CLUSTER statement to name the variables that identify the first-stage clusters, which are also called primary sampling units (PSUs). The combinations of categories of CLUSTER variables define the clusters in the sample. If there is a STRATA statement, clusters are nested within strata. If your sample design has clustering at multiple stages, you should specify only the first-stage clusters (PSUs) in the CLUSTER statement. PROC SURVEYFREQ assumes that each cluster that is defined by the CLUSTER statement variables represents a PSU in the sample, and that each observation belongs to one PSU.

If you use a REPWEIGHTS statement to provide replicate weights for BRR or jackknife variance estimation, you do not need to specify a CLUSTER statement. Otherwise, you should specify a CLUSTER statement whenever your design includes clustering at the first stage of sampling. If you do not specify a CLUSTER statement, then PROC SURVEYFREQ treats each observation as a PSU.

If your sample design includes unequal weighting, use the WEIGHT statement to name the variable that contains the sampling weights. Sampling weights must be positive numbers. If an observation has a weight that is nonpositive or missing, then the procedure omits that observation from the analysis. See the section Missing Values for more information.

If you do not specify a WEIGHT statement but include a REPWEIGHTS statement, PROC SURVEYFREQ uses the average of each observation’s replicate weights as the observation’s weight. If you do not specify a WEIGHT statement or a REPWEIGHTS statement, PROC SURVEYFREQ assigns all observations a weight of one.

To include a finite population correction (*fpc*) in Taylor series variance estimation, you can input either the sampling rate or the population total by using the RATE= or TOTAL= option in the PROC SURVEYFREQ statement. (You cannot specify both of these options in the same PROC SURVEYFREQ statement.) The RATE= and TOTAL= options apply only to Taylor series variance estimation. The procedure does not use a finite population correction for BRR or jackknife variance estimation.

If you do not specify the RATE= or TOTAL= option, Taylor series variance estimation does not include a finite population correction. For fairly small sampling fractions, it is appropriate to ignore this correction. See Cochran (1977) and Kish (1965) for more information.

If your design has multiple stages of selection and you are specifying the RATE= option, you should input the first-stage sampling rate, which is the ratio of the number of PSUs in the sample to the total number of PSUs in the study population. If you are specifying the TOTAL= option for a multistage design, you should input the total number of PSUs in the study population.

For a nonstratified sample design, or for a stratified sample design with the same sampling rate or the same population total in all strata, you can use the RATE=*value* or TOTAL=*value* option. If your sample design is stratified with different sampling rates or population totals in different strata, use the RATE=*SAS-data-set* or TOTAL=*SAS-data-set* option to name a SAS data set that contains the stratum sampling rates or totals. This data set is called a *secondary data set*, as opposed to the *primary data set* that you specify with the DATA= option.

The secondary data set must contain all the stratification variables listed in the STRATA statement and all the variables in the BY statement. Furthermore, the BY groups must appear in the same order as in the primary data set. If there are formats that are associated with the STRATA variables and the BY variables, then the formats must be consistent in the primary and the secondary data sets. If you specify the TOTAL=*SAS-data-set* option, the secondary data set must have a variable named _TOTAL_ that contains the stratum population totals. If you specify the RATE=*SAS-data-set* option, the secondary data set must have a variable named _RATE_ that contains the stratum sampling rates. If the secondary data set contains more than one observation for any one stratum, the procedure uses the first value of _TOTAL_ or _RATE_ for that stratum and ignores the rest.

The *value* in the RATE= option or the values of _RATE_ in the secondary data set must be nonnegative numbers. You can specify *value* as a number between 0 and 1. Or you can specify *value* in percentage form as a number between 1 and 100, and PROC SURVEYFREQ converts that number to a proportion. The procedure treats the value 1 as 100% instead of 1%.

If you specify the TOTAL=*value* option, *value* must not be less than the sample size. If you provide stratum population totals in a secondary data set, these values must not be less than the corresponding stratum sample sizes.