## Stratum Collapse

If there is only one sampling unit in a stratum, then
PROC SURVEYREG cannot estimate the variance for this
stratum. To estimate stratum variances, by default the
procedure collapses, or combines, those strata that
contain only one sampling unit. If you specify the
NOCOLLAPSE option in the
STRATA statement, PROC SURVEYREG does not
collapse strata and uses a variance estimate of 0 for
any stratum that contains only one sampling unit.
If you do not specify the NOCOLLAPSE option,
PROC SURVEYREG collapses strata according to the
following rules. If there are multiple strata
that each contain only one sampling unit,
then the procedure collapses, or combines, all these
strata into a new pooled stratum. If there is only
one stratum with a single sampling unit, then
PROC SURVEYREG collapses that stratum with the
preceding stratum, where strata are ordered
by the STRATA variable values. If the stratum with one
sampling unit is the first stratum, then the
procedure combines it with the following stratum.

If you specify stratum sampling rates using the
RATE=*SAS-data-set* option, PROC SURVEYREG
computes the sampling rate for the new pooled
stratum as the weighted average of the sampling
rates for the collapsed strata.

See the section "Computational Method"
for details. If the specified sampling rate equals
0 for any of the collapsed strata, then the pooled
stratum is assigned a sampling rate of 0.
If you specify stratum totals using the
TOTAL=*SAS-data-set* option, PROC SURVEYREG
combines the totals for the collapsed strata to
compute the sampling rate for the new pooled
stratum.

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