*The SURVEYSELECT Procedure* |

If you specify the option METHOD=PPS_SYS, PROC SURVEYSELECT
selects units by systematic random sampling with probability
proportional to size. Systematic sampling selects units
at a fixed interval throughout the stratum or sampling frame
after a random start. PROC SURVEYSELECT chooses the first unit
randomly from the entire stratum with probability proportional
to size 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). PROC SURVEYSELECT
uses a fractional interval to provide exactly the specified
sample size. The interval equals *M*_{h ·} / *n*_{h} for
stratified sampling and *M* / *n* for sampling without
stratification. Depending on the sample size and the values
of the size measures, it may be possible for a unit to be
selected more than once. The expected number of selections
or hits for unit *i* in stratum *h* equals *n*_{h} *M*_{hi}/*M*_{h ·} = *n*_{h} *Z*_{hi} . Refer to Cochran (1977, pp. 265 -266)
and Madow (1949).
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.