Current Population Survey Design and Methodology - Census Bureau

Appendix B.
Maintaining the Desired Sample Size
INTRODUCTION
The Current Population Survey (CPS) sample is continually
updated to include housing units built after the most
recent census. If the same sampling rates were used
throughout the decade, the growth of the U.S. housing
inventory would lead to increases in the CPS sample size
and, consequently, to increases in cost. To avoid exceeding
the budget, the sampling rate is periodically reduced
to maintain the desired sample size. Referred to as maintenance
reductions, these changes in the sampling rate
are implemented in a way that retains the desired set of
reliability requirements. The most recent sampling maintenance
reductions were implemented in 1999 and 2003.
These maintenance reductions are different from changes
to the base CPS sample size resulting from modifications
to the CPS funding levels. The methodology for designing
and implementing this type of sample-size change is generally
dictated by new requirements specified by the
Bureau of Labor Statistics (BLS). For example, the sample
reduction implemented in January 1996 was due to a
reduction in CPS funding and new design requirements
were specified.
MAINTENANCE REDUCTIONS
Developing the Reduction Plan
The CPS sample size for the United States is projected forward
for about a year using linear regression based on
previous CPS monthly sample sizes. The future CPS
sample size must be predicted because CPS maintenance
reductions are gradually introduced over 16 months and
operational lead time is needed so that dropped cases will
not be interviewed.
Housing growth is examined in all states and major substate
areas to determine whether it is uniform or not. The
states with faster growth are candidates for maintenance
reduction. The post-reduction sample must be sufficient to
maintain the individual state and national reliability
requirements. Generally, the sample in a state is reduced
by the same proportion in all frames in all primary sampling
units (PSUs) to maintain the self-weighting nature of
the state-level design.
Implementing the Reduction Plan
Reduction groups. The CPS sample size is reduced by
deleting one or more subsamples of ultimate sampling
units (USUs) from both the old construction and permit
Current Population Survey TP66
U.S. Bureau of Labor Statistics and U.S. Census Bureau
frames. The original sample of USUs is partitioned into
101 subsamples called reduction groups; each is representative
of the overall sample. The decision to use 101 subsamples
is somewhat arbitrary. A useful attribute of the
number used is that it is prime to the number of rotation
groups (eight) so that reductions have a uniform effect
across rotations. A number larger than 101 would allow
greater flexibility in pinpointing proportions of the sample
to reduce. However, a large number of reduction groups
can lead to imbalances in the distribution of sample cuts
across PSUs, since small PSUs may not have enough
sample to have all reduction groups represented.
All USUs in a hit string have the same reduction group
number (see Chapter 3). For the unit, area, and group
quarters (GQ) frames, hit strings are sorted and then
sequentially assigned a reduction group code from
1 through 101. The sort sequence is:
1. State or substate.
2. Metropolitan statistical area/nonmetropolitan statistical
area status.
3. Self-representing/non-self-representing status.
4. Stratification PSU.
5. Final hit number, which defines the original order of
selection.
For the permit frame, a random start is generated for each
stratification PSU and permit frame hits are assigned a
reduction group code from 1 through 101 following a specific,
nonsequential pattern. This method of assigning
reduction group code is used to improve balancing of
reduction groups because of small permit sample sizes in
some PSUs and the uncertainty about which PSUs will actually
have permit samples over the life of the design. The
sort sequence is:
1. Stratification PSU.
2. Basic PSU Component (BPC)
3. Original hit number.
The state or national sample can be reduced by deleting
USUs from both the old construction and permit frames in
one or more reduction groups. If there are k reduction
groups in the sample, the sample may be reduced by 1/k
by deleting one of k reduction groups. For the first reduction
applied to redesigned samples, each reduction group
represents roughly 1 percent of the sample. Reduction
Maintaining the Desired Sample Size B−1