*The SURVEYSELECT Procedure* |

The method of systematic random sampling (METHOD=SYS) selects units
at a fixed interval throughout the sampling frame or stratum
after a random start. PROC SURVEYSELECT chooses the first unit randomly
from the entire stratum and then treats the stratum observations
as a closed loop. This is done to obtain an unbiased variance
estimator, as suggested by Lahiri (Murthy 1967). If you specify
the sample size (or the stratum sample sizes) with the SAMPSIZE= option,
PROC SURVEYSELECT uses a fractional interval to provide exactly
the specified sample size. The interval equals *N*/*n*, or
*N*_{h}/*n*_{h} for stratified sampling. The selection probability
for each unit equals *n*/*N*, or *n*_{h}/*N*_{h} for stratified sampling.
If you specify the sampling rate (or the stratum sampling rates) with
the SAMPRATE= option, PROC SURVEYSELECT uses the inverse of the rate
as the interval for systematic selection. The selection probability
for each unit equals the specified rate.
Systematic random sampling controls the distribution of
the sample by spreading it throughout the sampling frame or stratum
at equal intervals, thus providing implicit stratification.
You can use the CONTROL statement
to order the input data set by the CONTROL variables before
sample selection. If you also use a STRATA statement, PROC
SURVEYSELECT sorts by the CONTROL variables within strata. If
you do not specify a CONTROL statement, PROC SURVEYSELECT applies
systematic selection to the observations in the order
in which they appear in the input data set.

Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.