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