|Sorting by CONTROL Variables|
If you specify a CONTROL statement, PROC SURVEYSELECT sorts the input data set by the CONTROL variables before selecting the sample. If you also specify a STRATA statement, the procedure sorts by CONTROL variables within strata. Sorting by CONTROL variables is available for systematic and sequential selection methods, which include METHOD=SYS, METHOD=PPS_SYS, METHOD=SEQ, and METHOD=PPS_SEQ. Sorting provides additional control over the distribution of the sample and gives some benefits of proportionate stratification.
Control sorting is not available when you use a SAMPLINGUNIT statement, which defines groups of observations as units (clusters) for sample selection. See the description of the SAMPLINGUNIT statement for information about ordering clusters before systematic or sequential selection.
When you specify a CONTROL statement, the sorted data set replaces the input data set by default. Alternatively, you can use the OUTSORT= option to name an output data set that contains the sorted input data set.
PROC SURVEYSELECT provides two types of sorting: hierarchic serpentine sorting and nested sorting. By default (or if you specify the SORT=SERP option), the procedure uses serpentine sorting. If you specify the SORT=NEST option, then the procedure sorts by the CONTROL variables according to nested sorting. These two types of sorting are equivalent when there is only one CONTROL variable.
If you request nested sorting, PROC SURVEYSELECT sorts observations in the same order as PROC SORT does for an ascending sort by the CONTROL variables. See the chapter "The SORT Procedure" in the Base SAS Procedures Guide for more information. PROC SURVEYSELECT sorts within strata if you also specify a STRATA statement. The procedure first arranges the input observations in ascending order of the first CONTROL variable. Then within each level of the first control variable, the procedure arranges the observations in ascending order of the second CONTROL variable. This continues for all CONTROL variables that are specified.
In hierarchic serpentine sorting, PROC SURVEYSELECT sorts by the first CONTROL variable in ascending order. Then within the first level of the first CONTROL variable, the procedure sorts by the second CONTROL variable in ascending order. Within the second level of the first CONTROL variable, the procedure sorts by the second CONTROL variable in descending order. Sorting by the second CONTROL variable continues to alternate between ascending and descending sorting throughout all levels of the first CONTROL variable. If there is a third CONTROL variable, the procedure sorts by that variable within levels formed from the first two CONTROL variables, again alternating between ascending and descending sorting. This continues for all CONTROL variables that are specified. This sorting algorithm minimizes the change from one observation to the next with respect to the CONTROL variable values, thus making nearby observations more similar. For more information about serpentine sorting, see Chromy (1979) and Williams and Chromy (1980).