PROC SURVEYSELECT: STRATA Statement :: SAS/STAT(R) 9.22 User's Guide

The SURVEYSELECT Procedure

STRATA Statement

STRATA variables </ options> ;

The STRATA statement names variables that partition the input data set into nonoverlapping subgroups (strata). The combinations of levels of STRATA variables define the strata. PROC SURVEYSELECT then selects independent samples from these strata, according to the selection method and design parameters that you specify in the PROC SURVEYSELECT statement. For information about the use of stratification in sample design, see Lohr (2009), Kalton (1983), Kish (1965, 1987), and Cochran (1977).

The STRATA variables are one or more variables in the DATA= input data set. These variables can be either character or numeric, but the procedure treats them as categorical variables. The formatted values of the STRATA variables determine the STRATA variable levels. Thus, you can use formats to group values into levels. See the discussion of the FORMAT procedure in the Base SAS Procedures Guide and the discussions of the FORMAT statement and SAS formats in SAS Language Reference: Dictionary.

The STRATA variables function much like BY variables, and PROC SURVEYSELECT expects the input data set to be sorted in order of the STRATA variables.

If you specify a CONTROL statement, or if you specify METHOD=PPS, the input data set must be sorted in ascending order by the STRATA variables. This means you cannot use the STRATA option NOTSORTED or DESCENDING when you specify a CONTROL statement or METHOD=PPS.

If your input data set is not sorted by the STRATA variables in ascending order, use one of the following alternatives:

Sort the data by using the SORT procedure with the STRATA variables in a BY statement.
Specify the NOTSORTED or DESCENDING option in the STRATA statement (when you do not specify a CONTROL statement or METHOD=PPS). The NOTSORTED option does not mean that the data are unsorted but rather that the data are arranged in groups (according to values of the STRATA variables) and that these groups are not necessarily in alphabetical or increasing numeric order.
Create an index on the STRATA variables by using the DATASETS procedure (in Base SAS software).

For more information about BY-group processing, see the discussion in SAS Language Reference: Concepts. For more information about the DATASETS procedure, see the discussion in the Base SAS Procedures Guide.

Allocation Options

The STRATA options request allocation of the total sample size among the strata. You can specify the total sample size with the SAMPSIZE= option in the PROC SURVEYSELECT statement. When you request allocation with the ALLOC= option in the STRATA statement, PROC SURVEYSELECT allocates the total sample size among the strata according to the allocation method you name. You can request proportional allocation (ALLOC=PROP), optimal allocation (ALLOC=OPTIMAL), or Neyman allocation (ALLOC=NEYMAN). See the section Sample Size Allocation for details about these methods.

Instead of requesting that PROC SURVEYSELECT compute the sample allocation, you can directly specify the allocation proportions by using the ALLOC=(values) option or the ALLOC=SAS-data-set option. Then PROC SURVEYSELECT allocates the total sample size among the strata according to the proportions you specify.

By default, PROC SURVEYSELECT computes the allocation of the total sample size among the strata and then selects the sample by using the allocated sample sizes. If you specify the NOSAMPLE option, PROC SURVEYSELECT computes the allocation but does not select the sample. In this case the OUT= output data set contains the stratum sample sizes that are computed according to the specified allocation method. See the section Allocation Output Data Set for details.

You can specify the following options in the STRATA statement after a slash (/):

ALLOC=name

specifies the method for allocating the total sample size among the strata. The following values for name are available:

PROPORTIONAL | PROP: requests proportional allocation, which allocates the total sample size in proportion to the stratum sizes, where the stratum size is the number of sampling units in the stratum. See the section Proportional Allocation for details.
OPTIMAL | OPT: requests optimal allocation, which allocates the total sample size among the strata in proportion to stratum sizes, stratum variances, and stratum costs. See the section Optimal Allocation for more information. If you specify ALLOC=OPTIMAL, you must provide the stratum variances with the VAR, VAR=(values), or the VAR=SAS-data-set option. You must provide the stratum costs with the COST, COST=(values), or the COST=SAS-data-set option.
NEYMAN: requests Neyman allocation, which allocates the total sample size among the strata in proportion to the stratum sizes and variances. See the section Neyman Allocation for more information. If you specify ALLOC=NEYMAN, you must provide the stratum variances with the VAR, VAR=(values), or the VAR=SAS-data-set option.

ALLOC=(values)

lists stratum allocation proportions. You can separate values with blanks or commas.

Each allocation proportion specifies the percent of the total sample size to allocate to the corresponding stratum. The number of ALLOC= values must equal the number of strata in the input data set. The sum of the allocation proportions must equal 1.

Each allocation proportion must be a positive number. You can specify each value as a number between 0 and 1. Or you can specify a value in percentage form as a number between 1 and 100, and PROC SURVEYSELECT converts that number to a proportion. The procedure treats the value 1 as 100% instead of 1%.

List the allocation proportions in the order in which the strata appear in the input data set. If you use the ALLOC=(values) option, the input data set must be sorted by the STRATA variables in ascending order. You cannot use the DESCENDING or NOTSORTED option in the STRATA statement.

ALLOC=SAS-data-set

names a SAS data set that contains stratum allocation proportions. You provide the stratum allocation proportions in the ALLOC= data set variable _ALLOC_.

Each allocation proportion specifies the percent of the total sample size to allocate to the corresponding stratum. The sum of the allocation proportions must equal 1.

Each allocation proportion must be a positive number. You can specify the value as a number between 0 and 1. Or you can specify the value in percentage form as a number between 1 and 100, and PROC SURVEYSELECT converts that number to a proportion. The procedure treats the value 1 as 100% instead of 1%.

The ALLOC= data set should contain all the STRATA variables, with the same type and length as in the DATA= input data set. The STRATA groups should appear in the same order in the ALLOC= data set as in the DATA= data set. The ALLOC= data set is a secondary input data set. See the section Secondary Input Data Set for details. You can name only one secondary data set in each invocation of the procedure.

ALLOCMIN=n

specifies the minimum sample size to allocate to any stratum. When you specify ALLOCMIN=n, PROC SURVEYSELECT allocates at least n sampling units to each stratum. If you do not specify the ALLOCMIN= option, PROC SURVEYSELECT allocates at least one sampling unit to each stratum by default.

The minimum stratum sample size n must be a positive integer. The ALLOCMIN value n times the number of strata should not exceed the total sample size to be allocated. For without-replacement selection methods, the ALLOCMIN value should not exceed the number of sampling units in any stratum.

COST

indicates that stratum costs are included in the secondary input data set. Use the COST option when you have already named the secondary input data set in another option, such as the VAR=SAS-data-set option. You provide the stratum costs in the secondary input data set variable _COST_.

A stratum cost represents the per-unit cost (the survey cost of a single unit in the stratum). Each stratum cost must be a positive number. Cost values are required if you specify the ALLOC=OPTIMAL option.

COST=(values)

specifies stratum costs, which are required if you specify the ALLOC=OPTIMAL option. You can separate values with blanks or commas.

A stratum cost represents the per-unit cost (the survey cost of a single unit in the stratum). Each stratum cost must be a positive number.

The number of COST= values must equal the number of strata in the input data set. List the stratum costs in the order in which the strata appear in the input data set. If you use the COST=values option, the input data set must be sorted by the STRATA variables in ascending order. You cannot use the DESCENDING or NOTSORTED option in the STRATA statement.

COST=SAS-data-set

names a SAS data set that contains the stratum costs. You provide the stratum costs in the COST= data set variable _COST_.

A stratum cost represents the per-unit cost (the survey cost of a single unit in the stratum). Each stratum cost must be a positive number. Stratum costs are required if you specify the ALLOC=OPTIMAL option.

The COST= data set should contain all the STRATA variables, with the same type and length as in the DATA= input data set. The STRATA groups should appear in the same order in the COST= data set as in the DATA= data set. The COST= data set is a secondary input data set. See the section Secondary Input Data Set for details. You can name only one secondary input data set in each invocation of the procedure.

NOSAMPLE

requests that PROC SURVEYSELECT allocate the total sample size among the strata but not select the sample. When you specify the NOSAMPLE option, the OUT= output data set contains the stratum sample sizes that PROC SURVEYSELECT computes. See the section Allocation Output Data Set for details.

VAR

indicates that stratum variances are included in the secondary input data set. Use the VAR option when you have already named the secondary input data set in another option, such as the COST=SAS-data-set option. You provide the stratum variances in the secondary input data set variable _VAR_.

Each stratum variance must be a positive number. Stratum variances are required if you specify ALLOC=OPTIMAL or ALLOC=NEYMAN.

VAR=(values)

lists stratum variances, which are required if you specify ALLOC=OPTIMAL or ALLOC=NEYMAN. You can separate values with blanks or commas.

Each stratum variance must be a positive number. The number of VAR= values must equal the number of strata in the input data set. List the stratum variances in the order in which the strata appear in the input data set. If you use the VAR=(values) option, the input data set must be sorted by the STRATA variables in ascending order. You cannot use the DESCENDING or NOTSORTED option in the STRATA statement.

VAR=SAS-data-set

names a SAS data set that contains the stratum variances. You provide the stratum variances in the VAR= data set variable _VAR_.

Each stratum variance must be a positive number. Stratum variances are required if you specify ALLOC=OPTIMAL or ALLOC=NEYMAN.

The VAR= data set should contain all the STRATA variables, with the same type and length as in the DATA= input data set. The STRATA groups should appear in the same order in the VAR= data set as in the DATA= data set. The VAR= data set is a secondary input data set. See the section Secondary Input Data Set for details. You can name only one secondary input data set in each invocation of the procedure.

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